Another site probably won't solve the underlying problem that Who is Hiring addresses - the problem of non-YC companies posting jobs on HN. Basically, Who is Hiring provides an outlet for what would otherwise be spam. Move it off to another site and then non-YC companies will still want to post on HN because that's where the eyeballs are.

The companies posting in Who is Hiring aren't posting on HN because they have no place else to post.