I think the reason no one uses Prolog is simply because generalized backtracking problems (i.e. the problems you would use Prolog for) are too computationally expensive and infrequent enough that:

a) You would never train staff or hire experts on Prolog for such few problems and...

b) You could probably do much better with a heuristic algorithm anyway. Especially on hardware of the day when Prolog came out, but even today I imagine this holds true.

Prolog may be useful for quick exploration of a problem space. But for code that needs to run frequently, there just isn't a great need for what Prolog has to offer.