Because it was a tiny little pea-sized speck of Grace ok, that's why. Give the poor thing a minute.

A few seconds is all it takes; there's a snapping sound like legos clicking into place and all of a sudden Castiel's nose doesn't look so broken anymore. There's still blood all over his face, down his shirt, but he's sitting up with a bewildered look on his face, a hand pressing against his chest as if he doesn't quite know what happened.