Pigs have a lot of physiological similarity to us (thus why fetal pig dissections are often done in high school/undergrad biology). Plus there are fewer ethical

Pigs have a lot of physiological similarity to us (thus why fetal pig dissections are often done in high school/undergrad biology). Plus there are fewer ethical concerns around using an animal we already use for food. I imagine there's also a consideration for ease of raising. These pigs had to be kept separate from others and tested frequently for disease because of having some aspects of their immune system weakened to reduce rejection risk. This would probably be easier to do ethically for pigs compared to primates.