Just like with all weight classes that have been added, even back to when the UFC brought back the LW division there are always guys fighting out of place, meaning not in their optimal weight classes. There are guys all over the world fighting at fly that could make straw and if the UFC added the division they would then have a reason to make the move. Happens every time. WW dropped to LW, LWs dropped to FW, FWs dropped to BW, BWs dropped to fly, Flys will drop to straw. Put the right letters on the gloves and it's amazing how public perception changes. "Cans" or nobodies all the sudden get super powers and turn into legit fighters in public eyes because of 3 small letters. It's magic.
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.