It's such a big ocean that the dilution factor is enormous. People think of it as all the same place because all we see is the flat level surface. Whales have needs of certain specific areas. It's not like they can be moved off into some other area and they will be able to survive in it just fine. Different whales have very different food requirements.
Gray whales are bottom feeders.
Blue whales are krill feeders. Humpbacks, however, are opportunistic, feeding mostly on fish in the northern hemisphere and krill in the Antarctic. Humpbacks and blues have been observed in the same area in California eating krill together. Different whales found in the same general region are often doing very different things, harvesting different levels of the food chain, and feeding in different physical habitats. Most whales are specialists.