Most people support creators on Patreon or OnlyFans because they want to support the creator and that is a way they can do that.
Now, as far as what platforms creators choose, much of that comes down how well designed the site is and how much in fees the site takes out.
For me, Patreon takes about 10% and OnlyFans takes about 20%. Most other sites I've looked at take between 30% and 45%.
One of the other two main reasons I've chosen Patreon especially is because I'm treated as a Creator, just like everyone else using the site. A lot of other paysites for models treat us more like objects or employees.
The other main reason is that they're not promoting other, more explicit content on my page. I don't do the really explicit stuff, so I can't compete with that. I also don't fit into the inked or alt girl genre so sites that focus on those things don't work for me.
So for me to choose an alternative, I'd need a well designed sites with solid features (especially an Android app), no more than 20% in fees, a site known to possible fans, and no one advertising on my page of the site. I'd also want to be treated like the small business owner that I am.