Mind you, if you say it like that, then 40K is just as off-the-wall. We're all just jaded by twenty-five years of exposure.
Sci-fi has always been more fragmented than fantasy, at least in the eyes of the general public. "Fantasy" is basically Tolkienesque orcs, elves and dwarves ina European medieval setting. Sometimes you get a bit of Conan-style sandals & sorcery creeping in. With Sci-fi, the closest things you've got to a "default" look is Star Trek and Star Wars, which are both fairly different from each other. On the one hand, it makes it easier to do something unique, because there's not such a juggernaut taking up the mental space, but on the other, its harder to do anything "generic" to maximise sales.
Iron Kingdoms is basically "trad" fantasy moved from the middle ages to the industrial revolution. It's the same sort of playing with fantasy tropes and cliches that 40K did. Given the popularity of both, there must be something appealing in that.
