That is another huge determining factor. The big cost is laying the infrastructure. The kind doesn't matter so much. So, if you are doing new deployments, fiber is more likely. The cable company here is all FTTH all the time for new build outs. However once that shit is deployed a replacement is a lot of money that you'd rather not spend. So they are less inclined to do it.
Well new developments also tend to not be low income. Usually middle and upper class is what they target. No surprise then that is where you see more of it.
There are plenty of rich neighbourhoods where I live with no fibre. The one right next to me is a good example. About 2 blocks away, and they have the same cable and DSL offerings I do in my cheap condo. Neither the telco nor cable company feels there's enough money to be made in ripping up and redoing the lines in either place, despite the fact that those houses are almost all 7 figures.
Go out in to a new subdivision though, and it is usually FTTH.
Also when they do rip things up and replace, of course they target the rich places since those people are more willing to spend the money. Offer someone low income the option of $100/month gigabit or $20/month 1.5mbit and they will likely go with the 1/5mbit. Ya it is way more per bit and annoyingly slow on the modern Internet, but it gets the job done and $80/month is a lot in the budget of someone low income.