The reason? Long range mages can harass them and farm easily and can push the lane much harder towards your tower. Note that it's very hard to farm near your tower (Yes I know about the mechanics of the tower. For melees, attack the melee once, let the tower hit them twice and AA once. For ranged, hit it once, let the tower hit it, and hit it again). The problem is that the tower doesn't always hit the minion that you want to kill, and you get screwed over on CS when a wave of your minions come and try to help.
The biggest offender here is the adorable Fizz. Max his E first. Sure, I can farm more easily near the tower. Whoops, wasted a lot of mana and my potential escape. Max his W first; better last hitting. Oh ****, Lux just used her bind and AoE slow spell when I try to last hit that low HP minion for the spike damage and grabs the first kill. Yes, I know boots and three potions help. However, it's really hard to try to dodge the skillshots and AoE spells AND try to last hit at the right time at the same time. I'm saying that Fizz has a very terrible skillset that would allow the melee assassin to farm safely AND effectively. Heck, even I think Talon has better farming potential than Fizz (and his Rake when ranked a lot wastes a LOT of mana).
Now there are some melee assassins like Katarina and Akali do have great farming potential. However, this is because they don't use mana when they use their skills and are limited by their cooldowns (in the case with Akali, energy). And these champions are meant for newbies anyways.
My point being is that I fail to see why ppl choose to play AP melee assassins in the mid lane when they can get harassed and pushed easily by long range mages. Heck, even short range mages like Annie, Ryze, and Kennen can **** Talon, Akali, and Fizz over in a long shot. Sure, they are supposed to COUNTER them because most of them are gap closers. However, their farming potential is very bad there since they get pushed hard. If you want better counter to long range mages during the laning phase, fight fire with fire. Fight the long range with long range.
I'm planning on getting Fizz to counter the long range mages and AD carries. However, I think Fizz is better top lane since most folks at top are melee like Fizz.