Not exactly the underdogs among the ranks of video game protagonists, space marines are nevertheless an oft-maligned crew. Sure, designing a game about marines in space shooting aliens doesn't take a whole lot of imagination, but we keep buying them, don't we?
And besides, wouldn't Mario just look adorable in a little space suit? He'd be like Captain Olimar, but with a sick 'stache. Only Mario wouldn't have an army of Pikmin to back him up when the enemy comes calling; he'd have to survive with his assault rifle, his space ship, and the dog tags of his fallen comrades hanging 'round his thick, hairy neck. Yeah, he's perfect for this one, we know.