The real question si why does everyone else have so few types?
We get 2 real 24th century Romulan types, 2 Klingon, and 2 cardassian. A little variety would have been nice.
They're not the heroes, therefore there's not as much demand for toys and model kits of those ships. Combined with the fact that back in the day, it cost a lot of money to build new models for filming. It surprises me that they actually built a new model for the Cardassian ship in The Wounded, considering it was only in the first 5-10 minutes of the episode.
When they switched to CG, things got a little odd. Nearly every Enterprise episode to feature Klingons introduced a new design each episode, while the Romulans ended up with an alien ship of the week design from Voyager.