It's the trend that is going on with the game, the vast majority of characters being released have great mobility tools in their kit while at the same time not losing in damage or utility to older champions that doesn't have any kind of escape/gap closer. Braum for instance offers a ton of utility to his team while also having a fairly big jump. The older champions that retain their popularity are pretty much the ones that already had great mobility, such as Leblanc, which also have one of the very best bursts in the game, stellar roaming and wins her lane pretty brutal. In terms of quantity and quality she offers as much as, say, Morde or Malzahar, which is a good mid laner that I really enjoy but is absolutely not viable in the competitive scene as far as I know, you can't expect the best players in the world to not capitalize over Malza while he simply stands there supressing one target while doing the same to himself.
This is a huge problem. Of course LB, Morde and Malzahar are three different kinds of mages with different strengths and weaknesses (not so much for Malza and Morde though, they share many weaknesses especially lack of mobility), but the point is, there is no reason whatsoever to pick the latter two over LB at the highest levels of this game as their clear weaknesses can't be circumvented by... anything. No big play will save your ass because you have no tools to do so other than flash (which the mobile champions also have over their already built in mobility). The game is starting to feel really, really divided between two groups of champions: the one with mobility and the ones without it. The last ones can't even dream to offer stuff that justifies their pick, except pretty much for Xerath (his range is so big that, as people use to say, it's almost a form of mobility as he can stay way distant in the back line) and Syndra (because she does what no one else could do, a stun with a super long ass range. Also a great lane bully and a master of pickoffs. Not sure about today's Syndra though, she too got nerfed).