It's because Nostr is new. Not many people know or have heard about Nostr.

The second problem is that Nostr isn't a website or app. You have to do this strange thing of using a nsec and people are used to using passwords and centralized websites.

It will take time but Nostr will win.

No one is making relays pay. The free relays are beating the paid relays. It's up to the relays administrator.