(Image credit: Al Bello/Getty Images)

The highest-paid U.S. athlete is not endorsed by Nike, nor does he own a clothing line.

Boxing champion Floyd Mayweather, Jr. was named the highest-paid U.S. athlete for the second year in a row, but it shouldn't be a huge surprise, said Stephen Greyser, professor emeritus at the Harvard Business School and an expert in the business of sports.

Mayweather, 36, was paid $90 million in the last year, none of it coming from endorsements, according to Sports Illustrated.

"I earn a lot of money but the money is not what motivates me," Mayweather said in a statement. "I work very hard at my craft and appreciate all the fans that support my efforts by buying my fights. Being at the top of my sport for over a decade validates my willingness to train hard, fight hard and give everyone something to enjoy. It is a honor to be the highest paid athlete again this year but it is just a part of my love for the sport of boxing and what it has done for myself and my family."

Coming in second in Sports Illustrated's list is LeBron James, who is competing in the NBA playoffs with the Miami Heat. He earned $56.5 million in total, $17.5 million of which was his salary and $39 million of which came from endorsements. James has a few basketball shoes named after him included in his deal with Nike plus endorsements from Coca-Cola, McDonald's and Samsung.

So how is it that Mayweather surpassed James by $33.5 million?

First, Greyser pointed out, compensation for boxers is not particularly high.

"Only the absolute top tier of boxers are able to cash in significantly," he said.

Mayweather clearly fits that bill. He received the highest guaranteed purse in the history of boxing with his last fight at $32 million. He also is his own fight promoter through his own company, Mayweather Promotions. He also has fought successfully in not just one but three weight classes.

Second, Greyser said, there are so few elite boxers in "such a distinctive sport, and the championship bouts are relatively infrequent."

There is a reason why the Olympics are also lucrative, as they take place every two or four years.

It helps that boxing competitions are also often hosted in large arenas that charge high prices for tickets. The same could be said for other sports, like football, but boxing events are also pay-per-view spectacles.

"All of these things represent revenue streams," he said. "The consequence is top tier boxers are able to generate very big revenue for their work."

That's a good thing, because, with many exceptions, boxers haven't historically had the highest endorsement value.

Mike Tyson, former world heavyweight champion, now has made a name for himself in films and speaking engagements. But in his heyday he did time for a rape conviction. And who could forget the infamous ear-biting incident?

Read more: Mike Tyson on Veganism and Evander Holyfield's Ear

Greyser said boxers "don't fit with the images most companies are looking for in endorsed athletes."