Whitebeard is already an old and dying man during the battle that killed him. He had taken a blade to the chest which undoubtedly worsened his condition, but he was still duking it out with Akainu and winning. We aren't dealing with Prime Whitebeard vs Netero here. Whitebeard just doesn't have the feats to do anything in the space between eating attacks that can stop whatever he's trying to do.