Has anyone considered that perhaps the Ambassador was simply an unsuccessful design? Or that there was a policy shift in Starfleet's shipbuilding; hence the 2nd gen Excelsiors?
Maybe they suffered an unusually high attrition rate, either through a series of disastrous exploration missions or the Cardassian & Tzenkethi wars? It's possible there were only ever meant to be a handful of Ambassadors built anyway and it was just a stopgap because the Galaxy project wasn't moving forward as quickly as expected.
That last one might make some sense as the Ambassador does seam like a bit of the throwback the the old Connie and Excelsior, rather than a logical progression. Fair evidence that the design was made in a relative hurry from tried and true technology, not meant to push the envelope so much as fill a void.
Maybe they were built for a war with the Klingons that never came and when relations improved the line was terminated and resources reallocated to other ships.