The game is insanely snowbally now, thanks to the changes to Mejai's, Dragon, early game relevance and other factors that determine the outcome of a game in the first 20 minutes or so. We see Katarinas, Lee Sins, Kha'Zixes, Fioras, Yasuos, Zeds, Fizzes, Akalis and Rivens because they want to snowball/roam in the early game (Akali/Kata is post 6 though) and get an early dragon. Mages that are immobile are nearly out of question because not roaming isn't a good strategy right now, and the other ones still usually need some ramping up until mid game, unlike the assassins that bully out quite early and snowball extremely heavily. Then we have the occasional Morgana ban, but she goes support to let her carry snowball, so it is still kind of relevant and Warwick, whose brokenness is not really due to his snowball potential but due to challenging smite.
This isn't just about Mid and Top lane about roaming and killing. Supports are pretty much similar. Sustain is something that is getting less and less viable, because kill lanes are all that matters. Supports are always Leona, Morgana, Braum, Thresh and Blitzcrank, sometimes a Janna or a Sona or a Nami, but Taric and Soraka just can't really give the carry the first blood he needs to secure an early dragon and turn the tables in the game.
Anyway, mobility is a thing, early game is a thing, snowball is a thing, and assassins have all of that (plus quite a few of them don't even need the blue buff, allowing the jungler to stay in the harder jungle a little longer assuming he has a sustain skill that is kind of costy, like GP or Yi), while most old school mages are just outshined unless they are bullies themselves or have long range to kite their enemies.
Can't say I like the game how it is, though :C