There's really no knowing why the Greeks chose liver, or some specific Greek chose liver.

It could have been because the liver was regenerative and they were using the titan with the regenerating liver as another reference to humanity, or it could have been as someone pointed out because predators like to eat the liver first, or it could have been that you can take damage to the liver and live for hours afterwards in agony whereas the same amount of damage to the heart or lungs is apt to cause death to come much quicker.

Perhaps there are undiscovered variants out there were the eagle or a serpent or some other animal attacks his genitals, or his eyes.

But at any rate the Eagle ate his liver, it means nothing in particular, and at the same time it means every particular interpretation we can place on it.

Whatever the actual reason for choosing the liver was, we can be certain it wasn't its regenerative property, since such proprerty was unknown to the Greeks at the time.