Remember this ended with a drawing. Some states, for purposes of minors gambling, consider even raffles gambling. Really with 20 grand on the line and ESPN on the hook most states will find a way to consider it gambling, just for the chance to confiscate the money and look tough.
You may be right (and probably are especially since it would allow greedy governments to confiscate wealth), but I just thought that to be legally considered gambling, there had to be some money at risk by the participant.
I’m just guessing here, but I think the bigger reason for excluding minors is that ESPN likely requires winners of contests such as these to allow their names, likenesses, etc. to be used by ESPN for promotions. That might be problematic for minors since such contracts are likely not binding upon minors. Like I said, though, that’s just a guess.