I forget, didn't Drake attack that island and try to take it over?
I'm pretty sure Drake isn't interested to getting himself territories.
It was so obvious. The death of both Ace and Whitebeard, and the changes of the world that occured after Whitebeard's death. All because Shanks wasn't on time to help Whitebeard. All that would be avoided.
Yes, Luffy did a miracle thing to get Ace from excution, but it would cost the life of Whitebeard and many pirates who followed Whitebeard. While I'm pretty sure without Luffy and having Shanks at Marineford. All the casualties on both sides pirates and marines would less to say the least.
But because Kaidou has a grudge against Whitebeard, and try to kill him. Shanks has no choice, but to stop Kaidou.
It seems Drake blame Kaidou for that, so he went to look for him.