I don't know if this is the actual reason, but I can see value in a country protecting it's food supply to ensure that long-term, they stay self sufficient. Subsidies that keep production local would be a means to that end.
We don’t care about being self-sufficient in avocado production, so why do we care about corn so much? I feel like the self-sufficiency is a post-hoc rationalization and the real reason has more to do with politics and lobbying.