It's the same reason why so few people sign up to pay for YouTube or Twitter. Video content is available elsewhere. Whatever the hell Twitter content is can be found just by talking to people.
On the other side so many do pay for Spotify and Netflix because that content is locked down pretty well. You can pirate some things, but it's a pain. Paying is easier.
Reddit seems to think it has a product that has value. Like so many web content hosts before it, it's probably wrong. It probably can't work as a business unless it's showing people adverts. And the problem with that model is that people hate adverts.
Honestly, until users realise that they can't have a content host unless they pay, no sites like Reddit will survive long term.