I don't know what happened, but it's easy to imagine ways it could have happened. It might not have anything to do with difficulty. First off, YoYo probably came in with drills the guys were unfamiliar with. Having just had great success with Darby's stuff and wanting that same plan, vets may have gotten frustrated or questioned the usefulness of the new drills vs the old drills. "This worked great last year's way, why change now because of this guy who hasn't trained an Elite 8 team?"
Maybe he doesn't have the command of Darby and isn't able to convey why the drills are useful, or can't control 20 year old kids which is no easy task.
If Darby was great (as you say), then having someone come in who wasn't as great would be sensed instantly. Any time someone new arrives and is in a position of power, they are always tested for their boundaries like the raptors testing the fences.
Finally, the 2 guys in the most powerful seats were unused to being there, and didn't know how or want to lead a team to greatness. It's a lot of responsibility, and if you don't truly have the desire to be great, others watching you will see that and follow the lead. Not only did they not lead the team down the right path, they tried to take the easy way out and it was a terrible example for the young guys in the formative weeks of their intro to college bball. When there's no clearcut leader, the actions of those with the most power become magnified. If Blake showed up every day motivating everyone else, it is easy to fall in line and follow his lead. Without him providing energy, everyone settled for just doing enough to get by. I've seen it happen many a time.