The spread offense can work in the NFL, but not one where the QB runs it like Tebow does. There are many version of the spread and I think some of them can work, at least as a reliable and successful gimmick play.
One of the biggest reasons for running the spread is for getting favorable matchups on offense. In the NFL, the matchup differences between a WR and a LB (or strong safety) is less than in college...at least that's one of my theories.
Also, for some reason I still don't understand, the shotgun isn't that popular in the NFL, and I think the majority of spread offenses are run from the shotgun and not from under center.
Oh, and I just thought of another reason a friend said to me recently. The defensive schemes are more complex in the NFL and therefore harder to read. So trying to run the zone read option will be hampered greatly, for example.