I think it depends on the region but, from my limited experience through friends, it was a mix between those wanting the stipend that they got for having a foster child, and the super strict religious ones that believed that more discipline was the answer to everything.
Foster care is a lottery. You could end up with an amazing family, but you could also end up with in a new abusive environment. I have a few friends that were in foster care and from their experience, I'd say that the amazing families are sadly a minority in the system.
But if anyone wants to adopt, they have to basically prove that they are a closer to perfect home, and put down a fortune in admin fees in a process that takes years.