not all phones have huge developer support.. with a big selections of ROMs.
the phone must be popular.. to attract developers.. more developers, brings more talented developers.
also, the OEM has to release the orgial codes so that developers can use them to create from. only a few are able to create code from scratch (very very seldom). the better the OEM support the more developers can do.
like i said before.. when you mentioned that you got a backflip... it was never really that popular.. because it was from att.. and att kind of messed it up some with their own requirements/changes on it. it was not really adopted by the development community. you can tell by how much activity is in each phone's sub-forum.
ex: EVO sub-forum is very active vs. Tranform