Bishops have a great deal more mobility than knights. A knight in the wrong corner takes several moves to catch up to a distant passed pawn. A bishop needs far

Bishops have a great deal more mobility than knights. A knight in the wrong corner takes several moves to catch up to a distant passed pawn. A bishop needs far fewer. Generally the trade off is a bishop pair, where you do have all the squares covered, vs the slower knights, who are also the only pieces that can threaten a piece without being threatened back. The advantages are highly position dependent, with closed games largely going to the knights and open games going to the bishops pair. But most games turn into open games at some point, so statistically the bishops end up better.