Why him? He was mooted as a possible Hal Jordan when Warners was trying to get Justice League to the screen. And he may well have impressed the studio enough in Terminator Salvation to give him his shot yet.
Why not? The John Stewart Lantern was black, but Jordan is typically white in the comics. Fanboy reaction to colour-blind casting hasn’t always gone over well (see: Michael Clarke Duncan in Daredevil), but that seems a poor reason not to give him a shot. True, he’s never carried a film, but he was decent in his small Wanted role.