Top Lane champs like Irelia and Yorick are so strong now versus low mobility mages that they just pick mid and destroy.
Champs like Malzahar simply can't hold them back because the damage they put out combined with the tankiness of their kit prevents Mana using Mages from being able to succeed.
Eventually, they wear out and have to recover mana or risk getting their tower bashed in.
There's no way to trade damage either because Irelia and Yorick are overwhelmingly strong 1 v 1 players.
Low mobility mages have no escapes. So these guys can use their ults to lock us in every time. Flash only works that first time, then as soon as their ability comes up they can dive and enclose with almost no risk and no counter. There's not enough damage that we can do 1 v 1 to stop it.