Probert's original Ambassador design was quite different to the Enterprise-C, although I'm not sure if I like it better.I suspect the continued unabashed fuglyness of Ambassador class kept Excelsior's on the screen for quite awhile. But I don't know.
Supposedly the Ambassador model was damaged after it was last used in Emissary as the Yamaguchi. I don't think there was a time where an Ambassador was called for after that. The Excelsior model was used for the Lakota because it was a larger, more detailed model and the Ambassador was already damaged by then.
After that, Greg Jein built a new Excelsior model for Flashback which then appeared a few times subsequently. Then it was pretty much all CGI, and they never had to build an Ambassador, so it never appeared again.