I like the option of being able to focus on the core competency of the primary ability scores. I like the high cap. That is one of the greatest strengths of 5E. You don't HAVE to opt into a bunch of alternative abilities and can focus on the basics. It seems like anyone who wants the cap at 18 wants to force others (not themselves, as they have their own free will) to diversify with their feats, removing the option to focus on the core competency.To force people to use more feats.
Note that forum uses are heavily self selected on the side of wanting more complex mechanics.
So consider this post representing a large but quiet group of people who just want a simple +2 Str on their barbarian instead.
(Even though I personally would rather have more interesting feats).
I have more than one player in my life that just want to be good at their baseline abilities, and don't want to deal with decision paralysis of hunting down the specifics of 3 different feats to remind themselves of other things they can do.