If you believe the stories out of Carolina, while he was "off" there, the coaching staff knew how to push his right buttons and get him to buy in. Rivera is a stronger willed coach than Garrett.
And that's really the hallmark of some great coaches.......... the ability to handle guys who just weren't great fits. Garrett's management style seems to be that he has only one way to get something done and if you don't immediately buy in, see ya. That's a terrible way to run a football team. Look at what it got Chip Kelly in Philly.
No sure why you've decided Rivera is stronger willed than Jason Garrett, but since there's zero evidence to support that argument either way, I'll leave it alone.
Hardy, if you weren't paying attention, had his share of problems when he was in Carolina, too. With the exact things that appear to have gotten him out of favor in Dallas. His teammates have been very supportive of his effort on the field and on game day in both places. Jason Garrett himself has been very positive about Greg Hardy's effort in practices and in games. It's the rest of the time where he struggles.
That hallmark of great coaches is *not* the ability to handle guys who aren't great fits. Great coaches assemble great players who fit. The league is littered with great coaches who cut bait on great players when they're deemed to be more trouble than they're worth.
Greg Hardy's problem was that he didn't correct any of the things that got him into trouble in CAR. That's the reason the Cowboys don't want him back, and it's the reason why a very talented pass rusher in his prime is still sitting around his house this late into free agency.
If you're take were right, there are several great coaches in this league who'd step right up and handle this guy whose only problem is he's not a great fit and just needs a little tough love. They're not stepping up.