It's because hairless females don't lactate normally, or don't lactate at all. Therefore the babies(if they survive and aren't killed off by mom) will be small and thin. I believe they also have a much bigger chance of miscarriage and internal hemorrhaging(uncontrollable bleeding). It's just in the rat's best interest not to ever breed them. There are really no positive outcomes for the mom or the babies.

I had a female hairless get bred on accident, she had 8 healthy babies, but only 2 survived. They barely survived on what she could produce for them milk-wise, but when they started eating on their own they grew much better. I still have one of the babies(she's around a year old now, named Emily) and she's doing fine(has a mammary tumor) but is fine

Having a hairless male as a breeder would be fine, and the resulting babies would carry hairless. You could safely breed them back to their dad or another hairless male, and get some nekkids. I love hairless beebs, so sweet. Like a living hot water bottle.

I would advise that if you do get a hairless male for a breeder, to either get him a normal haired friend or keep back one of his sons as a permanent buddy. He needs someone who has normal fur to keep him warm. Hairless have a much faster metabolism to keep themselves warmer than normal rats. Mine have no problems maintaining their weight on my mix and lab blocks.

Hope that helps!