I think it just came down to how Avery Brooks carried himself after shaving his head and growing his beard back out. And those who remembered Brooks's role as 'Hawk' on
Spenser: For Hire and its brief spinoff,
A Man Called Hawk, it reminded them of that character (a certified BAMF). Brooks naturally kept a bare head and a goatee before landing the gig as Sisko, but producers encouraged him to grow his hair out and shave the beard so that 1) viewers wouldn't immediately associate him with Hawk, and 2) they already had a bald captain on TNG, and that was a big deal at the time; they didn't want another, the fact he was black was a big enough deal all on its own.