Playing devil's advocate a bit here, but:
Maybe it's true that the impetus ultimately lies with the player, but isn't it possible that two-person carts just don't make it evident how to use them properly? Single-person carts would certainly eliminate much of that.
For example, the correct way to use a two-person cart is to drive up to one ball, drop that person off with some clubs, and drive to the other one, right? The problem is with the people who drive to one ball, wait for the green to clear, hit that ball, and then drive to the other, certainly, but in a single person cart there would be no reason to do it the incorrect way. The person in cart B isn't going to wait with the person in cart A, they're going to scooter over to their own ball. Shouldn't that speed up play?
Or we could simply teach people how to use a regular cart correctly.
To paraphrase........2-person carts don't cause slow play, people cause slow play.