Steemit is a fairly niche audience for now. Celebrities will target their on-line presence where they can reach the largest audience of the right demographic. Making a few dollars (or even thousands) here may not be enough. I saw that happen on Tsu where they signed up some celebs at the start, but they didn't stay. It's a bit of a chicken/egg problem, but Steemit can grow organically to a point where it's interesting for them.