Originally Posted by Helmight
The problem with having a thief who's also a League champion is that thieves, generally speaking, aren't into fighting. Their goal in life is to steal what they can and get out, without being seen if possible. A true thief, therefore, would have no desire to join an institution where they'd be FORCED to fight near-legendary beings to the death. And it's not like there's anything for them to steal by participating in the League, so they have literally no reason for joining. The thieves we've seen that have become champions all have been motivated by an external factor - the "something else" that you've mentioned. If, for example, Vi had never fallen out with her criminal buddies, she never would have become a champion because she'd still be stealing things on the streets. If Twisted Fate had never received magical powers, he'd still be running cons with Graves. It's unreasonable to expect that a thief who's still a thief would enjoy battling with things like Cho'Gath and Kog'Maw on a daily basis.
I actually have a League Judgment that would cover Ritz's reason for joining the League. Long-story-short, she gets blackmailed into fighting in the League.
A small figure in a purple summoner’s robe walks purposefully down the hallowed halls of the Institute of War. With the hood covering their face, it is impossible to tell if the figure is male or female. A group of summoners pass the small figure, and the figure greets them with a simple nod. None of the summoners suspect that the small figure is anything but what it appears to be. The figure continues onwards up a flight of stairs.
Left, left, right, left...no, wait.
The figure pauses at the junction as it reaches a hand inside the robe and retrieves a crumpled sheet of paper. A young girl’s face is briefly revealed as the figure leans forward to peer at the hand-drawn lines that represent the hallways of the Institute of War. Ritz looks up and around to get her bearings, then tucks the map away.
Ritz makes a right, navigating the hallways with precision. Minutes later, she stops in front of an ornate door. She doesn’t need to check to make sure she’s got the right door. Ritz’s eyes move up the door until they reach a nameplate that reads: “High Councilor Vessaria Kolminye”. The girl permits herself a small grin of satisfaction as she picks the lock and slips inside.
Unfelt by the young thief, a magical pulse ripples through the air. Only one person appears to notice it: an older yet still beautiful woman in a purple summoner’s robe. Instantly, she glances upwards, in the direction of the pulse. After a beat, she smiles, an altogether more sinister smile than that of the girl.
As the door swung shut behind her, Ritz gazed around the massive office of the High Councilor. Bookshelves lined the walls, each packed with what look to be books on magic. A large desk occupied one side of the room, stacked high with papers. A larger window dominated the opposite wall, letting in the light from the setting sun.
As she peeled off the summoner’s robe, Ritz let out a low groan.
"This is gonna take a while."
Carelessly dropping the robe at her feet, the girl set to work, moving to the desk to begin the search. A quick check is enough to tell the thief that what she’s looking for isn't on top of the desk. Ritz opened the desk drawers one by one, hoping to find her prize, but without any luck.
Her lips tightening in frustration, the young thief looked around the room, searching for another hiding place. As she turned back and forth, Ritz’s arm caught one of the stacks of paper on the desk, knocking it over and sending papers swirling around the room.
Ritz leaned over to pick up the fallen papers, but stopped as she caught a glimpse of metal on the desk. Her earlier grin began to resurface as she brushed away the remaining papers to reveal a small safe built into the desk. Ritz shrugged, brought out a candy bar and a pocket knife, and set to work.
The candy bar was only half-finished by the time the safe swung open. To Ritz's curiosity, the safe wasn't protected by any magical seal or booby-trap, although she would have been prepared for either. Inside the safe’s plush interior was a single item: an unmarked box smaller than Ritz’s palm. Reaching out a hand, Ritz flipped the box open, and grinned.
The object of her search, a ring carved from a single purple gem, sat inside. Ritz already had a buyer, a mage from Demacia, who was offering a sizable reward for the ring. He had even been so generous as to provide the route to the High Councilor's office. The money from the reward would be enough to feed an entire village for over a year, and there was one on the eastern border of Demacia she had in mind.
Ritz plucked the ring from the box and slipped it into the pouch at her belt, exchanging it with a small wooden effigy of herself with its tongue stuck out - the gesture was clearly meant to be insulting.
But at the very moment that she took one step from the desk, a voice sounded from the now-open door.
“And what do you think you’re doing?”
Instinctively, Ritz snapped her gaze to the door to find High Councilor Vessaria Kolminye glaring at her. Had the High Councilor entered minutes earlier, Ritz might have been intimidated.
With a jaunty wave, the Swift Thief dashed for the window with unnatural speed, an unseen wind propelling her like an arrow.
“I think not.” The High Councilor’s voice was as cold as the mountains of Freljord. The summoner raised her hand and with a burst of energy, Ritz yelped as she was sent flying back towards the desk. Just before Ritz would crash into the desk, the force propelling her stopped, and the girl was dumped unceremoniously to the ground.
Ritz looked up at the High Councilor and, in a fit of pique, hurled the half-eaten candy bar at the summoner. Instinctively, High Councilor Kolminye’s hand rose to block the perceived attack, and Ritz used that opportunity to make another dash for the window and escape. The speed at which she ran now rivaled that of a bolt of lightning.
Mere inches away from the window, the High Councilor’s magic caught her again, this time suspending her in the air. The High Councilor swept her hand down, slamming Ritz into the ground and sending the stolen ring sliding from her still-open pouch.
Vessaria picked up the ring as she said, “So you’re the so-called Swift Thief. Ritz, was it?” The High Councilor twirled the ring in her fingers almost absentmindedly. “Now what shall I do with you...?”
Ritz made another attempt to get up, but the High Councilor’s magic prevented her from making a single move. The young thief scowled; she had always disliked mages, particularly summoners, and this one was making her look like a fool.
Abruptly, the High Councilor appeared to make a decision, and Ritz was dragged up to dangle at the High Councilor’s eye level. Ritz quickly swung a fist at Vessaria, trying to do anything to disrupt the sorceress' concentration, but she was out of reach. Vessaria acted as though Ritz had done nothing at all.
As Ritz swung around, trying to land even a glancing blow upon Vessaria's nose, the High Councilor said, “A vagabond as adored by the public as you would be ill-suited to a prison cell, would you agree? Of course, you needn't be sent to our dungeons if you're willing to accept an... alternative."
Ceasing her futile struggling, Ritz said, "You mind putting that in words, lady?"
"Very well. I am sure you are familiar with what we do here, child. And the League of Legends is always in need of new Champions to fight. I would have you join them as a Champion yourself."
The proposal wasn't at all what Ritz had expected. And even though she was no slouch with a knife, going up against some of Runeterra's most powerful Champions wasn't an appealing idea to her.
Seeing her hesitance, Vessaria added, "Of course, if you'd prefer a cell, I can arrange that."
Things were a lot simpler now. Prison was never an option for Ritz. Too many were depending on her to come back.
"Tch, fine lady. I'll fight a few matches."
High Councilor Kolminye smiled, though the smile didn’t reach her eyes. “Oh, I think you'll be in for more than a few, Swift Thief." Turning away from Ritz, Vessaria added, "But I am not so unreasonable as to forever keep you. I'll make you another deal: if you can ever manage to take this ring from me, you can go free. How does that sound?”
"Heh, deal! I'll be out in a week, tops!"
"We shall see."
With a wave of the High Councilor's hand, Ritz rose up shortly, and then was launched back with a yelp, flying out the door. As she fell to the floor and skidded across the tiles, Ritz quickly bolted upright to watch as the doors to Vessaria's office began to close.
A mere second before they did, a small object flew out at Ritz. The half-eaten bar of candy struck the thief's forehead, sending her back to the floor. She lay in a daze as the doors clicked shut.
With no one to observe her, High Councilor Vessaria Kolminye smirked at the ring in her hand.
“We shall see, Ritz,” the High Councilor murmured.
Dropping the ring, she watched as the spell holding the ring together faded, and the shaped glass crumbled to dust before it hit the floor.
Vessaria chuckled briefly. Everything had gone perfectly.
I think that'd cover the reasoning quite nicely.