I think the whole thing with only the pearl and the dutchman fighting was because both sides knew that if the pearl couldn't take down the dutchman, no one could, and if the pearl was taken down, then the pirates were even more fucked than before. I actually liked that they kept Calypso out of the fight except for the maelstorm because that made it more about ship fighting and not who some goddess chooses to win or lose. Also, Barbossa kept saying something about how Calypso made things fair back when she was not bound and it seems that's what she did with the maelstorm, made it so that just the pearl and the dutchman could fight and neither really had an advantage.