Because there isn't talent for it.
5'2 athletes are quite rare outside of Asia and 3rd world countries.
The only notable straw I can even think of is Rambaa Samdet and IIRC he's fought at fly before and should if he plans on making a serious MMA career.
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.