Maybe they just never built very many to start with. Therefor you don't see them for the same reason you don't see the newly created Sovereign Class, at the time of the Dominion War there are only two or three of them.
This is what I mentioned on another topic. If only 6-12 were ever built (like Galaxies and Constitution), and few like Enterprise and Horatio destroyed, you woudn't have many around. When Dominion War came, they would have tried to complete Galaxies that already had frameworks built and in storage rather than go with building Amabassadors from scratch.