I really don't know when things started to change, but once upon a time you could customize every program in every way. Now many applications don't even let you change the damn font size. Something something "single user experience" something something but I don't know why.
As for Twitter specifically, they made one stupid change years ago which made it much much harder for new users to find new people to follow - and therefore have any clue just what the hell the point of twitters is - and now they wonder why it's hard for new users to find people to follow. For ONE MILLION DOLLARS IN CONSULTING FEES I will tell them what that mistake was. That change, too, could be customizable. It once was. And then it wasn't.