There's more to the Fed dominance than simply being the good guys. Fed ships, as a majority, are easier to make and work with. They're also more fleshed out and have countless reference material. All this, coupled with most modelers' lack of free time/experience/patience/insanity, adds up to a crapload of the blue boys being created.
Romulans and Jem'hadar ships are notoriously organic, fluid, and complex in their design and construction. Even the hi-res CGI artists tend to stray away from them, partially for these reasons. I do agree that there should be more aliens, at least more Klinks and Roms, but I don't see things changing anytime soon - if ever.
Romulans in particular, are something of a white (green?) whale for me since regardless of how much I like their designs, it takes a lot of time and vision to build one. Part of making a model is figuring out just what to do and how to do it - kind of like putting a puzzle together except you not only have to make the puzzle, but the pieces as well.
Romulans are more molded than assembled, and everything tends to flow together which makes it difficult to work with. Feds, on the other hand, have clearly defined parts and you can see where everything connects. The same goes for the Klinks, though their textures are what tend to turn most away from tackling one of their ships.