You only get a 46 man roster for the game, and they decided to carry only 4 WR's. It only makes sense that he would have been left off, since the others had much more experience and familiarity.
If he manages to stick around for the next season, I suspect that Michael Floyd will have a nice bounceback season. The Erhardt-Perkins offense is an excellent fit for him, and he'll have the best quarterback in the league, and possibly of all time, tossing balls to him.
Link: http://www.espn.com/nfl/story/_/id/18625776/michael-floyd-healthy-scratch-new-england-patriots-super-bowl-li