I'm gonna bet one of the reasons the WMMA Champions are pretty in active is they're all pretty old.
Zhang - 36
Val - 37
Kayla - 35
Zhang and Val have been around forever, and it's going to take it's toll. Kayla less so, and she's the youngest, but her weight cut is famously brutal.
The best hope for more frequent title defences is probably to get a younger, more active champion, but even then the UFC's scheduling of top ranked fighters and champions is pretty bad. Each champion should defend twice a year on average, IMO, more if they're dominant and healthy (Merab, for example), less if they get battle scars - but it's never going to happen.