The National Football League is starting its own YouTube channel and Major League Baseball, which began streaming games online 13 years ago, bought the rights to manage the National Hockey League’s Web operations. But action sports — including surfing, skateboarding and snowboarding — rule millennials’ viewing time on their mobile phones. Although Americans are still watching less video on mobile phones than traditional TV, action sports on mobile channels is a parallel universe of sports viewing and is likely to remain so.

According to The New York Times, the reasons are plentiful: young people have the limber bodies to accomplish skateboarding tricks and other action sports, viewers can watch and comment whenever they like (as opposed to traditional sports’ appointment TV viewing), and action sports’ “short bursts” are a better fit for mobile phone viewing.

NYT points out that, “digital media also tends to favor sports where people can upload their own videos, and one needs only to visit a ski slope, where children wear helmet-mounted GoPro cameras, or a skateboard park, where they follow one another with their iPhones, to see that amateur filming is a big part of these sports.”

Perhaps the biggest reason action sports flourish on digital media is that they aren’t encumbered by the big dollar contracts found in professional sports, giving YouTube channels content that is both cheap to produce and free of copyright restrictions and blackout dates.

YouTube chief executive Susan Wojcicki reports that her network has “more viewers on mobile phones alone than any cable network.” Although soccer is the most popular sport on YouTube, action sports is the next big category, with Red Bull’s action sports YouTube channel boasting 4.5 million subscribers. That puts the Red Bull channel “among the top three in the sports category,” says NYT.

“All these second-tier sports become much more the norm,” says Outrigger Media chief executive Mike Henry. “Major league sports are a bit more of a commitment.”