It helps if you think of Orcs in terms of a species instead of a race as we use the word race in the real world. Lets say hypothetically elephants were a playable race and they got +6 con due to their tough hide+body but -6 int. The stats here are an abstraction of what an elephant is, these are parts of the biological makeup of the animal. If you were to Tasha's rules the -6 int away at the expense of the Con because the elephant has the Enlightened Elephant Ancestry, you are erasing what makes an elephant an elephant, because part of what makes it what it is is the fact that it has a tough hide. The stat bonuses are an abstraction of some component of the makeup of the race that defines it as what it is.
