I find it weird that despite all three having historic seasons, Haskins is being looked at has an afterthought. Some articles don't even mention him. Murray is very good, though the case against him is very easy - he plays in the Big 12 for Oklahoma and I would throw for 3000 yards if I were their QB and I struggle to throw a spiral.
Setting aside Big 12 silliness, I think the answer has to do with the way narratives can harden.
The Heisman is a storybook award, for bad or for good. It goes to someone with a story. Ohio State's is complicated.
OSU spent most of the year as a disappointment. It's biggest failure was on a big stage against a so-so team, and the enduring image of Haskins was throwing again and again. He became the gunslinger doing what he could with a bad defense and not much running support, but it remained in the context of disappointment. The big non-conference win evaporated. And there were stats, but they always had that looming over them. Sadly, that doesn't get a guy a Heisman.
And in that, we probably underrated what OSU and him were doing. I had a friend who is an ardent OSU fan, one who is often levelheaded, and when the Michigan game came around, he said something like "If Michigan can't beat, THIS Ohio State team ..."
Of course THIS Ohio State team was 10-1, No. 8 in S&P. But they couldn't shake that feeling, even on here. As such, what should've been a big win in The Game became more of a referendum on Michigan. The BTTG didn't help, with a team destined to get loss No. 5.
Basically everyone, including many of us, treated OSU like a disappointment right up to the end, and by the time that could shift, there wasn't enough runway to fully throw things back into a positive gear. If something like the Michigan game happens 2-3 weeks earlier, I'd bet there's a better chance Haskins is contending and maybe there's a more real discussion about a playoff spot.
(It also brings into relief the situation in 2014, where OSU got the defining win at the start of November and benefitted from the Big 12's split/lack of a title game)