bb = big blind
BB = big bet
BB is still very commonly used today for historical reasons:
Back in the day, no limit hold'em was a rare game. Almost everyone played limit. In limit, you could only bet or raise 1 big blind on the flop, but on the turn or river, you could make a 2 big blind bet or raise! This 2 big blind bet or raise was commonly called a big bet (BB).
Everyone talked about their winrate in BB and when NL started to become popular, the terminology just stuck around.