...and the Romans started after the Greeks, and the North Amerindians had myths, too, as do the Inuit and the Chinese...
But they picked these three because more people are aware of them and therefore can relate to them. They'll probably have some other civs in the expansion pack, which, no doubt, is already in the mill. From what I've read and seen so far (not anything that moves, just static shots) it has the look and feel of a really great game. The subheadings of the 3 civs give the "feel" of 9 civs anyhow, so there's variation.
When you think about it, how many MesoAmerican myths does the average person know? How many have the least idea who Quetzlcoatl was, and/or what he did? The need in a game is to appeal to as large an element of the market as possible, while still keeping the unique flavor of the game. ES has a good track record and I submit that they will make another in a series of great games.
Bring on the frost giants, I say, let's see what they can do against the Titans!