Here's the "duh" reason:

Females lay eggs. Eggs mean babies. Babies mean $$. Pretty simple.

Some other reasons I came up with, based on my experience working at a pet store:
- Males seem to be more plentiful than females. I'm not sure if this is a fact overall, but it seems like we get wayy more males in than females. This may also be a result of breeders holding back more females so they can get more babies, and ditching the males because they may already have a few good, proven breeder males.
- Females get larger. In the pet trade, generally, a bigger snake gets bigger bucks.