I guess because the fantasy society is theoretical, it's easier to juxtapose an agenda onto it?

Oh yeah, definitely, that was pretty much the whole point of Star Trek. With fantasy/sci-fi, it's easy to construct an entire society devoted to justifying your Morality Anvil. Now that I think about it, C.S. Lewis kind of did the same thing - basically reconstructing BibleLand within a schema of European fairy tales (maybe to make it more accessible to the European Christian child who wouldn't be able to relate as much to the Bible's original Middle Eastern milieu?)