Cultural thing. I was watching an episode of mind field about freedom of choice and it opened up with talking about how bacon and eggs is the stereotypical go-to breakfast. When they looked into why that is, it was because a company that made bacon basically paid a bunch of doctors to say that people need to load up on a big meal in the morning to make their life better, and what better way to do that than to load up on some bacon!

So the real answer is marketing combined with time and people just doing it because that's what mom and dad did.