The Romulan military may not have had a great number of ships to begin with. They seem to always be about misdirection, giving an impression of a large military force of ships. Makes sense given their covert lifestyle. Why build a large force when you can make an enemy think you have one?
They seemed to be all on par, power and size wise, to Galaxy class ships. But we never see a great deal of them, even during the Dominion War.
It's very possible that after the heavy losses during the Dominion War, and possibly more losses with the supernova, there may only be a handful of D'Deridex class ships and the like left.
That would explain why we saw a TOS style ship than, say, a NEMESIS one. Or even one like Admiral Jarok used in "The Defector", though that was more a scout than an attack vessel.