First he cast Shatner. Not only he is bald under the toupee, but in the several instances (enough to turn it into a meme) he appeared shirtless, he had to sport a shaved chest.
I always figured Shatner's chest was to emphasize his pecs more, same reason bodybuilders do it. If the chest is too fuzzy, it sort of blurs everything and makes the muscles look less defined.
As for Sisko, I figured it was because Avery Brooks is much better looking with the bald head. He has the head/face for it.