Because Prolog and Lisp are really quite different. Have you used Prolog for a significant project? Its resolution and backtracking capability can be very powerful for some problems, and you'd have to implement them in Lisp.

I read the sources of an algorithm-derivation system that was done in Prolog (you give it some model relationships and it pattern-matches to find a learning algorithm to fit model parameters to data). It was quite elegant and got a lot of power for free.