(Edited for typos.) I saw a documentary once that looked at this. The Aztecs and Mayans only had stone tools. Therefore, cutting through the ribcage to get at the heart would have been very difficult. Since the priests were not concerned with the amount of damage they were doing, the easier path was to the come up from beneath the ribcage and go through the diaphragm to get to the heart. This is supported by the types of alters that they used for their sacrifices and the way the victims were bent over the alter (as depicted in the artwork these civilizations left behind). The way they are bent over backward exposed the belly so that the priests could easily cut their way through this way. The documentary was look at this in respect to a legend about Monctazuma having ordered a massive number of sacrifices for some celebration. They were trying to determine if it was really possible for them to have performed so many sacrifices in such a short period of time. For this they constructed two realistic mock-ups of human torsos and then had a heart surgeon try to remove the hearts with a stone knife. The surgeon found it almost impossible to cut through the ribs with that knife. However, he was able to cut through the diaphragm and get the heart out of the second torso in just a few minutes. So, they determined that the only way that Monctazuma's priests could have performed the required number of sacrifices was to do it in this way.