Pig hearts have been considered as a potential source of transplant organs for humans for several reasons. One of the main reasons is that the size and structure of a pig's heart is similar to that of a human heart. This makes it easier to transplant a pig heart into a human without requiring significant changes to the recipient's blood vessels and other tissues.

In addition, pigs are widely available and can be easily raised in large numbers, making them a convenient source of transplant organs. They are also relatively similar to humans genetically, which reduces the risk of transplant rejection.

While it is possible to transplant organs from other animals, such as cows, into humans, the success rates for these types of transplants are generally lower due to the differences in size and structure between the donor and recipient organs. Additionally, transplanting organs from other animals can carry a higher risk of rejection and other complications. These factors make pigs a more attractive option for organ transplantation.