Oh, I can tell you why.

It was two things. First is that a lot of programming is non-functional things like error handling and Prolog didn't get you good ways to reuse that code.

The second is that it's very easy for specific backtrackings to be exponential in nature, and given that when Prolog first came out, machines speeds were running less than 10MHz, it was very easy to make a small tweak in your program and have it take days to complete.