The issue you have with TT is that it's very low on the priority for Riot. While I won't deny that there is probably a good number of people who play on it (some of them solely on it), you have to understand that the majority of the people (a large portion, if I"m not mistaken) stick to Summoners Rift. And Riot's job is to cater to the masses, which means that they focus on Rift. Rift is also where a lot of the balancing for champions comes into play, which makes it an even larger priority for them.
Another issue is Treeline used to be the 'quick' game mode for people... now, if someone is looking for a quick 20 minute game, they go to Crystal Scar.
I have nothing against Treeline... but even if it gets changed around, I don't see many people switching back to it from the Rift. And that's the reason Riot won't make that a priority (especially when they have things like... I dunno, stealth remake? on their plate).