Calling fighting for the UFC 'the minros' is intellectually dishonest. There is not a sports organization in the world that can sell for $4,000,000,000 that is considered a minor league. This is literally, too stupid to discuss further.
Minor league MMA exists in the form of local, regional and lesser national orgs.
Regarding 185/205/265: the lack of signed fighters, high average age and demonstrably low skill and athleticism is a sourcing problem. There are plenty of other suitably sized high caliber athletes in other sports, including the minors of each, but we can't find them in MMA.
As a thought experiment, ask yourself a question, and answer honestly - if the starting pay for a heavyweight signed to the UFC went from 12k to get your head punched in, to 500k guaranteed, would more men signup to try?
Would a larger talent pool interested in heavyweight MMA produce a deeper division? Of course.