My understanding (and I am far from being any kind of expert on marine biology) is that, when orcas attack whales, they rarely try to eat the whole thing- they tend to attack baleen whales, and their target is one thing only: the whale’s big, meaty tongue. A pack of orcas will attack a baleen whale repeatedly, just to get into its mouth and start tearing out its tongue.
Once the tongue is gone, the orcas will swim off, leaving the mortally wounded, tongueless whale to drift off and die. The rest of the whale’s body is of no interest to the orcas, who leave the carcass for assorted sea scavengers.