Probably because there is a spacesuit available as a downloadable costume for the game. Which sort of sounds ridiculous as far as singing and dancing goes.
I know Riyo seemed to love it for some reason, I thought it might of started with him, but it seems it's an actual thing.
Perhaps Idolmaster_Xenoglossia had some influence on it as well, since at least one character wore a spacesuit in space in that series. The suits there looked different than these NASA style ones though.
