Actually, I don't think there are as many full conversion mods as there are 'expansion' mods. For example, there are more than a 100 mods (and a similar number of skins) listed on twc and while there are several full conversion mods, there are many others that just change army formations, unit sizes, build times and so on.

It certainly sometimes _appears_ as though every mod is a full conversion, but that's only because they're more talked about and they generate more interest. They have a lot more work into them, often with teams of people and their own forums, and this all translates into a deservedly higher community profile. But they're not the only fish in the sea.