Maybe the suitcase puts a magical sheild like thing around them so they can breath, or the little alien dude magic madeit possible. Also because why not?
Maybe that suitcase have it's own atmosphere? You know, like the TARDIS
...ohhhhhhh nice thinking-didn't think of that (as you can probably tell)
yaoi logic
LMFAO(⌒▽⌒)
WHY THE F*CK HASN'T HE SUFFOCATED YET? I MEAN HE'S IN GODDAMN SPACE!!!
(and I'm not taking yaoi logic as an answer... (=・ω・=))