Fems are better because the breeder can directly observe the female flower traits carried by both parents. This greatly increases the power of the breeder's selection. Also, both parents are potentially elites selected from many seeds.
Compare that to male selection for regular breeders. Virtually all commercial breeders, it seems, just rub some male stems, maybe flower them and pick the one with prettiest and most resinous male flowers. Some know how to reverse it to produce some female flowers but I don't think this is common. Then they chuck it. If you're lucky they'll run some seeds before selling to make sure it doesn't herm out but even that isn't guaranteed. I've never heard of a breeder doing is hitting the same clone with multiple males, growing out the seeds, and picking the male that produces the best female offspring. That's really the only way to know if your male carries good genes for female flowers. Nope, that takes way too much time and space ( == money).
So basically with regs you get a randomly selected male, selected mostly for vigor and a good stem rub. That's why regs aren't as good as fems in addition to being half male.