I think the main reason why so is so rare is that these cells hardly ever divide. Additionally the heart cells have little contact with carcinogens, compared to skin cells for example. Cardiac tumors can be caused by circulating metastatic cells (metastatic melanoma are an example for that), but this happens only rarely. The following thread on researchgate is also interesting: https://www.researchgate.net/post/Why_is_Cancer_not_observed_in_Heart