Those weight classes filled because there are a large amount of male human beings between the heights of 5'6 and 5'10.
In fact the overwhelming majority of them.
The same does not apply to 5'1 115lbers. This is why there is no Super Heavyweight division, because there are only a handful of men that are legitimate super heavyweights and most of them are in the NBA. You'd literally just be filling a division with the morbidly obese.
In fact I don't see anyone currently fighting at Fly that could make that weight outside of maybe guys like Useless Gomez. And he would probably win a title and it would be stupid because he's clearly not that good.
Useless is a very talented fighter that had a disappointing debut. IMO he can be a top 10 fly very soon. Talented fighter though.