Well the real reason is no alien races in Star Trek beyond the Federation have a lot of ship designs.
The Cardassians had three ships and two types of starbases. The Klingons have been using D-7 and Birds of Preys for hundreds of years. The Romulans just have the big bad warbird... and a little science ship. Nothing in between. It goes on and on. I know this is due to budget reasons, but it does make the other races look more than a bit two dimensional.