I think it is partly so hot because you know it would absolutely wreck Tony. You can't tell me he doesn't have a tiny sneaking desire to be a Jedi, and Steve all dressed up in the costume murmuring 'There is no peace, there is passion," would reduce him to a quivering pile of mush in about thirty seconds.

Also the costumes are quite flattering. Steve in the Darth Maul black robes? Fffff.