90% of superearths are discovered in close vicinity of their parent star, where existence of a moon is impossible due to tides from a star. In multi-planetary systems moons are almost impossible too, due to very close distance between planetary orbits. There are only few dozen systems that looks like our own (large distances, only 4 planets within 3 AU radius) - exomoons can exist only in that systems.
Good point most Exo earths have been found with M-class primaries, that would make it tough. Its just so hard to believe such a massive rocky world would be geologically stable, they must be hellish places or covered with water.