I think he had some deep-seated issues that started to surface after the 2014 title. For some people, reaching the pinnacle of your profession that early can be a bad thing. You spend your whole life working towards that high, only to realize that you're still going to have a lot of the same problems you had before. I think Hurley experienced something similar after his first title.
But the more straightforward answer is that there's a big difference between coaching a team and running a program. Ollie seemed to be pretty good at the former but way in over his head at the latter. It also didn't help that he was transitioning from coaching Big East recruits to AAC recruits at the same time Calhoun's players were being phased out. That was a double whammy - like having your training wheels fall off mid-ride.
I do think Calhoun deserves a lot of credit for 2014. He recruited the players and coached them for 1-2 years. When Ollie took over, they already had a lot of winning habits. By the same token, however, I do not think Ollie gets near enough credit for 2011. Yes he was a first year assistant, but his fingerprints were all over that team. That was the season Calhoun finally relinquished his attachment to the oversized lineups - that had for years clogged the floor and suppressed half-court offense - in favor of smaller alignments that were hugely successful. I have to believe Ollie had a lot to do with that change of philosophy.