Because I honestly am stumped as to what CAN be done about it. The fact is mobility in the form of blinks and dashes really isn't a problem when 1-2 people on a team have it, but when you have 3 or even all 5 champions on a team all being super mobile it really starts to become an issue. It's a compounding issue.
Give one of those champions a strong form of AoE CC (IE Malphite Ult) or a strong disruption ability (Gragas Ult) and there literally is nothing a team that is lacking mobility or abilities that disrupt positioning can do to properly manage it. Your only solution is to fight mobility with more mobility.
Because of this some champions are simply destroyed and, despite them actually being fairly strong, makes them almost totally unviable. The only really non-mobile champions you see frequently picked in several roles anymore are the ones with INSANE utility, slightly OP, or some amazing ability which lets them be useful without being mobile.
Take Ashe and Kog'Maw for example. Both of these champions see so much play despite their lack of mobility because one is really strong at peeling enemies off of herself (Ashe) and the other has such long range/ high damage he can safely stay out of harms reach before he kills his target (Kog.)
If you want to look at other roles check out Karthus. He has 0 mobility but he doesn't need to move around much to have any power because he has so much damage that just naturally occurs around him with Defile toggled on and the STRONGEST AoE SLOW IN GAME (which was honestly nerfed like crazy last patch.) He also has his passive which allows him to still be useful for several seconds after being killed and an ultimate which doesn't give a crap about your mobility when it hits everyone.
TL'DR Looking at it I honestly don't have a clue as to how it can be balanced out to make these champions competitive picks again without creating some anti-blink/dash item or buffing them in insane ways to fight mobility. You can't just flat out nerf all the mobile champions to hell just to make it fair on the non-mobile ones.