Winners write the history books. It's as simple as that.
By the logic of the show, Egyptian mythology (and really, their civilization) was created around the Goa'uld as gods and masters. So whichever Goa'uld the ancient Egyptians were actually exposed to would be the most prominent. Sokar and Cronus are the primary examples of Goa'uld that don't fit into the Egyptian mythology theme - but the first was banished and overthrown, and Cronus was only semi-prominent on the show and only then in later seasons, implying that Ra and the others were on the way up and Cronus was on the way out during their time on Earth.
Cronus also became a part of Greek mythology, and it's worth noting that Greek civilization didn't really start up until thousands of years after Egyptian. Even then, Cronus was a distant terror, one of the defeated Titans, and not one of the figureheads in the Greek pantheon. It could be theorized that Cronus actually did try to build up his own thing just across the sea from the Egyptian "gods", but found himself isolated instead.
So it's not that the Egyptian gods are the top positions, but rather that the leading Goa'uld became the Egyptian gods, and stayed in roughly similar positions of relative strength even after Earth revolted and was lost to them.