Because we all have busy day jobs and don't think this would be a good income opportunity
The problems with upgrading this forum is of different nature though (technical debt and difficult access to servers), and this could not really be solved by paying someone for it.
That said, I am currently moving/expaning some services over to a server rented by me and should the need arise to upgrade that server again (which I don't think will happen soon) I might open a Liberapay account to recover some expenses.