It’s actually because the mareep line’s skin grows thicker and more rubbery as they advance from one stage to the next. By the time they reach their ampharos stage, their entire body becomes covered in a thick, rubbery hide that’s perfect for insulating their internal organs from their own potent electrical powers.
As for why mega ampharos grows hair back … it’s because mega ampharos’s power is too great to be contained.
To clarify, its hair grows back to serve as electrical storage, not because it’s “too powerful.” There are scientific explanations behind this; this isn’t a shounen anime. —LH
You’re absolutely correct. We’re more of a monster collecting RPG with a shounen anime adaptation. —Bill
…what. —LH
The Pokédex According to Bill 