It wasn't just that rent was cheap; rent was cheap in many other places as well.

It's that rent was cheap in close proximity to the buyers and dealers artists needed to make a living, with a subway to get around on nearby, and (eventually) a critical mass of like-minded creators to hang around with.

This basically doesn't exist nowadays. You can move to western New Jersey to lower your bills, but it's a long haul to New York when you need to do business, and unless you get others to move with you, you'll be working in isolation or with a small handful of peers.