Prolog is an extremely small niche. Most of the questions are pretty basic and from students. I don't like seeing students being told odd things about Prolog, with a dismissive air of "Prolog makes no sense, you just have to feed it this nonsense to make it go." So another part of my motivation is to keep this sad religion alive and inviting. Haskell has a much larger community of people working very hard to make it inviting.
The hiring angle probably works better for other niches. Every Java EE question is answered by BalusC. I imagine that pays dividends for his consulting: he's the most helpful guy in that area and I'd hire him in a second if I were in charge of that kind of thing.
