My theory...
The Enterprise-B is the only Excelsior variant we see in all of Trek... until Paradise Lost, when we see the Lakota in action after an "extensive refit". Connection? I think, when the Ambassador was introduced, the Enterprise-B name was decomssioned in favour of the C, but was rechristened Lakota NCC 42---.
After all, the Enterprise-B would have been no more than 30 years old when the Ambassador-class was introduced. The original Enterprise served for 40 before being decomissioned, and the Enterprise-C was nearing 20 years old when it met fate at Narendra III.