From A Balancing Perspective, Creating A Non-ADC Meta Is Super Difficult
It's not like Riot wants it to be that way, it's just super hard for them to actually do something about it.
You either would have to nerf the support/marksman combination so hard that it is unplayable or buff something else so hard that it is super overpowered and outclasses all marksmen.
The last time they tried to change something about the stale botlane Meta, reworked
happened. To know how shitty this rework was, you just have to look at a list of Malicious Metal's posts.
I'd rather have marksmen in every game than have something like this ever happen again.