[quote=mdk]
I think mostly because, in terms of celestial bodies, moons are necessarily more common than planets. So it's reasonable to expect that there are (numerically) more habitable moons than there are habitable planets; and then, going by our own solar system and common-sense probability, most moons are probably orbiting the most massive planets, so gas giants theoretically work.Plus it totally aces the 'rule of cool,' so obviously, sci-fi is gonna go there.
[/quote]

Yeah, that's pretty much the main reason why. It certainly was for the Coyote series by Allen Steele. I need to get back into scifi reading.