IMO the #1 reason that girls give up softball at that age level is they just realize they are not that good at the sport. When a person is good at a sport they generally like to play that sport and stick with it. That is certainly not the only reason.
#2 is parents riding their a$$.
Probably the saddest reason is because of the parents. One girl in particular was constantly hearing comments from her dad in the bleachers. It didn't matter what she did, it was never good enough. She finally realized that the only way she could escape this was to quit the sport. I still don't think the dad realizes why she quit.
Once they get their drivers license whole world opens up to them too, seems like softball becomes less important. I am thinking great she can drive herself to softball lessons, what lessons?