People are just too damned lazy.

Nebula is small, it still only has a tiny fraction of the YouTube library, and you have to pay to access it up front.

Also, near non-existent marketing budget. People can't be expected to magically inuit the existence of an alternative platform. At some point, you need to advertise your existence and get your content creators to do the same.

The only creator I've ever heard mention Nebula even once was Abigail Thorn. And even she only talked about it during a couple of projects from years back.

Everyone who is anyone in the indie space just goes with Patreon instead.