Why do orcas eat only the tongue of whales, liver of sharks, etc.?
Killer whales have a wide range of prey. They hunt in coordinated packs called pods. They are very organized on a hunt and each has a specific job prior to the kill.

Where the confusion might be coming from is that different pods have different habits. It is true that Orca's have been witnessed eating only the tongue, this is not the norm. They tend to eat the tongue first because it's muscle, with lots of protein. Plus, they like the tongue, as we like a good steak. However, in most cases they will consume as much of a whale (typically baleen, or blue whales) as they desire. In fact, whalers hate them because they will follow whaling ships and wait for the fishermen to harvest a whale. They will then rush in and gorge themselves. Another case of them only eating the tongue is when they are well fed. In this case, they can afford to be picky and may only eat the tongue. Oh, by the way, the average tongue is the size of an elephant. So, it's no small meal.

As for sharks, it's pretty much the same scenario, however, there are many cases of Orca's only eating a great while shark's liver. The reason they target the liver (and they do it with surgical precision) is because it contains large amounts of a compound called squalene. Squalene is important to synthesis of certain chemicals in the Orca's body. They will also target the heart, as well. The attacks of great white's is a more recent trend among killer whale pods. They are quick learners and once they feel the benefits of these organs, they will then focus on the sharks when their bodies tell them they need the squalene. Of course, they don't know that. They just know they feel better and stronger after consuming GWS organs.

There may be pods out there that tend to have these habits (outlined above) more than others. Resident pods (staying in one area) seem to be calmer and are not as likely to have the tongue activity, but it is the residents that are often that which kill the GWS's for their organs.

Transients are far more aggressive than residents. They are likened to pack wolves and will do some extremely cruel things (by human standards). These are the one who are far more likely to eat only the tongue of a whale. It could be that since the shark organ habit hasn't been happening for very long, it's the residents who figured out the connection between health and these shark organs.

Another factor that could come into play is that scientists/marine biologists tend to believe that there are different species of killer whales. This may play a role in dietary habits as well.

Killer whales are the top (known) predator in the water. While a much larger whale, or GWS could potentially kill one, they are typically no match for a pod and will instead steer clear and only fight if no other alternative presents itself.

I hope this helps. I'm no expert, I've just done my research.