Have you ever applied for a job? Like a job, a company wants to sponsor someone so it will benefit them. It's advertising. Getting your name out there is the way to get noticed. Put yourself in the sponsors shoes...
"Why should I sponsor you and give you free gear based on an edited youtube video of your riding and not your contest (publicity) results, what does that do for me?"
Frankly, if I were a company looking to sponsor riders, the only videos I'd take seriously are the ones sent to my company directly, not posted on a message board or youtube only hoping someone will see it and it damn well better be the best of the bunch. I'd be willing to say half the kids wanting sponsored only want it for status, telling high school friends about it, and they are the only ones who will even care that you are. Way too many people are way too serious about it and don't just ride to have fun. When you start riding for yourself, that's when things happen, keep riding for status and you'll burn out.