One thing Earth has going for it is that it seems to be the by far most populated planet in the galaxy. Don't know how far back in time that would've applied, but most Goa'uld controlled civilizations appear small compared to ancient kingdoms and city-states.

So even with only millions of people, that could've been enough to have multiple distinct realms without direct conflict. They could also have been separated by time. Yu and Ra were contemporaries, but Apophis might not have been. We know that all the other System Lords are younger than Yu.

Or most of them didn't stay for long, they built their empires elsewhere, with slaves from Earth.