Mr Blaustoise implies that high mobility is played more frequently than less mobile champions because they are more fun.
I highly doubt this is the hade, but rather the fact that mobility champions are just better.
Players love to do well, we love to climb and will most likely play champions with a higher chance of carrying/winning.
I used to play Trundle a lot, he can duel most champions 1v1, that is if he can actually reach them. In todays meta it is almost impossible to stick to most popular champions because of mobility. If Trundle would get an ability that would make him stick to champions easily, like a chain preventing dashes, he would be really strong and would skyrocket in playrate.
Just look at Garen on the PBE with the on hit effects applying on his E. I've seen several streamers play him and having so much fun, not because mobility but because he is strong.
I would argue that all non-mobile champs would be played all the time if they would get abilities that could make them stick more easily to targets.
High mobility offers more options, initiates, ability to dodge skillshots and defenses, it is overall a better option and has way less counters than champions with no mobility. High mobility champions can also make comebacks, Katarina can be far behind but one roam bot could change it all. If you fall behind on a non-mobile champion like Darius there is no coming back, kited, CCd or just straight up blown up.
This is why they played more, higher chance of doing well and a higher chance of having fun and higher chance of a comeback.
But give low mobile champions a buff and i assure you nobody would play mobility champions if they wouldn't do well.
TL:DR. High mobile champs are not played more because they are by nature more fun, but because they have more options and more ways to do well.