It's all about bypassing regulations, just like Uber and AirBNB. Most US ISPs have old copper cables that only support DSL. Upgrading them means digging up the streets and that's expensive and a legal minefield. And those ISPs are local monopolies so why would they spend money just to keep the same number of customers who are locked in anyway?
I don't think that is very true in this day in age. Here in Cincinnati, the vast majority of houses now have fiber run to them. There are still some stragglers, but that's mainly because slumlord apartment owners don't feel like dealing with upgrades.