Size. Pig hearts are the same size as human hearts, cow hearts are larger, so easier to cut up for parts. Chimp hearts are usually smaller.

Also, the risk of transmitting zoonosis is larger in primates than in other mammals, because with humans being primates as well, more viruses/prions/fungi might be infectious to both.

Presumably because we already farm pigs and cows so there’s a supply chain and the ethics are ok in most people’s heads.

most likely because their (heart) valve anatomy is similar to humans ? this is just a guess btw.