People watch bowling. Pool. Pro Paintball. Once late at night I caught a demolition style "race" of trucks pulling campers (the drivers had sponsors).
It doesn't really take much of an audience to get sponsors. The key to lining them up is for the event organizers to make it clear to the sponsors who will be seeing their ads. If the spectaters interests and the sponsors are in agreement, then the deal works. I'd bet one could find sponsors for snail racing because there will be someone watching.
So I see no reason why computer games can't be either.