Well as much people like to complain, especially some in this thread the sad fact is that, girls just don't buy action figures. No, just because your niche community all bought the figure doesn't mean there's actual demand. I've seen numbers from stores and they shelfwarm. It's not marketing, toy companies have tried marketing to girls making doll versions where boys are the main demographic and they always fail.
The main people who buy female figures are the adult collectors which is why some lines like Marvel Legends can do a better job but the logistics in making female figures easy to find for people who want them and not overstocking is not an easy task. It's depressing since I hate having incomplete sets because they preferred to make c-tier male bad guy over a main character just because its a girl.
As a kid I always wanted an April figure but anytime I was at the store I was swayed over by getting my 30th raphael instead so I was part of the problem
I would hope that in the future more toylines including TMNT can do a better job though, keep releasing them even if they're rarer to see, I mean the problem is a toy company listens to internet whiners who most don't buy figures and make a wave featuring a female that doesn't sell and then we get no more female figures blaming hiring someone who didn't know how to read the market.
Also the sculpts that they give some of the faces of female characters, that's definitely not helping. Playmates being a huge offender for the most part through out the years.