Yeah, the Fire Nation's got the most advanced technology (not counting the random stuff the Mechanist was doing off on his own), but they don't have much of a landbase so presumably they don't have the same populations and natural resources as the huge sprawling Earth Kingdom. And we've already seen the Earth Kingdom itself is sort of misnamed, it's more of a sprawling mass of smaller kingdoms, so even once the FN's conquered one city-state they still have to start all over again with the next one...