Manager promotes all his employees except his top performer, who loses all motivation: 'He is hands down the best recruiter I've worked with among his peers.'

He did the one thing that was almost guaranteed to demotivate him. 

It can be difficult to stay motivated to work hard when it's been proven that hard work doesn't yield tangible positive results. If an employee works harder and performs better than all of their coworkers, they should be rewarded for that work, even if it doesn't come in the form of a promotion or a raise. Their boss could give them a higher title or a bonus to communicate that their work is valued and does not go unnoticed. That might work for a year, but if a top-performing employee gets snubbed for a raise year after year, you can't be all that surprised when they try to find a new job somewhere where they are valued more. They know their hard work is making the company a whole lot of money, and that there are companies out there willing to pay bigger bucks for hardworking employees. 

It's one thing if everyone works sort of hard, and nobody gets the raises they deserve. Obviously, that's a bad situation, but at least people aren't getting rewarded for doing nothing. But if the best employee doesn't get promoted and everyone else does, how is he supposed to see that as anything less than a slap in the face? Why would you reward the employees who performed worse than the employee who performed the best? You shouldn't do that unless you're actively trying to get rid of your top performer.