I think it may actually be due to the fact that other services are very regional, but if SE hosts it themselves then it's easier for people to access all the streams for all three minions. Last time there was only ONE incentive item and it wasn't based on the stream you purchased. But this time there is a different incentive item for every stream, so if SE hosts them all then they can make a page for purchase of the stream for each fan fest so that everyone has equal opportunity on the items.