I think it's the same reason that you replace Erlang with Go, because they both aim for concurrency. So in terms of writing ai programs, why can't Prolog be replaced by 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.