Bet on your beliefs
polymarket.com
Already showing over $694 million bet. I suspect the total wagered on the US presidential election will be over $2 billion worldwide. The fact that it is not legal in the US is essentially irrelevant as offshore books are plentiful and non-US bettors make up a significant share of the market.
In general, a single well-publicized event with months of futures betting will collect more revenue than a single sporting event, which just has a few days to collect revenue before the next game (and is competing with many other NFL games for dollars).