its not mobility nor cc creep
its more like a power creep aimed at the internal power of the kit mixed with its numbers
champions who have diwerse kits combined out of the strongest elements like mobility and cc are prefered ower the others just because they are stronger in game and compete more towards the wictory in organised play
guy like darius can win but he isnt as usefull as a charging sejuanig who just cc locked their adc
morde can win his game but he isnt as usefull as le blanc who can reach to enemy backline snap a carry and assist your team with snares
a very big part in this is played by balance of numbers vs kit owerload
even if champion has only one purpose and nothing more to it he cant be brought up to bar with sheer numbers due to all the bad interactions it causes, and there is the opposite, champions who have owerloaded diverse and usefull kits cant be starwer off that power in numbers because it would create similar bad interactions
which leaves us in enviroment where owerloaded champions cant pay for being owerloaded, while specialized champions cant realy excel at their area of expertise
riot creates more champions like this because they are fun
they are fun because they are effective
they are effective because they have better kits and dont pay for it