In order (numbers 1 and 3 could possibly be swapped out) :
1. Manziel tanked it at the end, not to mention the fact that he had already won it as a freshman. I think a lot of voters were a bit apprehensive to give it to him 2x in a row, as Archie Griffen was the only former player to win it 2x in a row, let alone as a freshman and sophomore.....but Manziel's tanking it helped Winston dramatically. That really made him the clear cut favorite for the last few weeks.
2. FSU's schedule was a complete joke. People made tons of comments (and they were certainly correct on some level) about OSU's lack of good opponents up until the last 2 games vs MSU and Clemson, but FSU's strength of schedule was even worse, and that was well documented. Their lack of any kind of quality opponents helped to allow Winston fattened statistics.
3. No competition....No one really rose to the challenge, including Mariota etc. Had (key word) Braxton Miller not missed 3-4 games due to injury and they not lost to Michigan State, he may have given him a much closer contest. Unfortunately, Miller missed 3-4 games and they did not get showcased in the title game, or things may have been somewhat closer. The fact of the matter is that J.Winston just didn't have much competition at all.