One way of looking at the history of programming language design is that there was a revolution with the introduction of the subroutine. Twenty or thirty years later, two refinements of the subroutine call were seriously considered:
I've just been programming in Prolog after a 20 year hiatus and realizing just how incredibly powerful unification and backtracking are. However, polymorphism won. Why?
A guess: message-passing was more easily tacked on to the then-popular practices and gradually absorbed. A gradual acceptance of Prolog's ideas would take a vehicle like Oz, only invented in the 90s, something like 20 years behind Smalltalk. Since Oz claims to support both procedural and logic programming in one clean package, I see no reason in principle the world couldn't have taken that path if it had known how at the right time. Instead the paradigm got tied to a more burn-the-diskpacks attitude and the 5th Generation disappointment.
(I haven't tried Mozart/Oz myself so far. I have played with Prolog.)
My experience with Prolog is that is works excellent when backtracking search is a good fit for your problem domain. However, if that is not the case much of the programming effort goes into fighting the backtracking search, bending it to ones own needs.
So my take on the situation is that backtracking search is too narrow a language feature to be generally useful. If we would have seen unification together with a more flexible search then we might have seen a different course of development.