It was an example of a more credible commitment than a blog post. To paraphrase Buffet's Noah principle, 'predicting rain doesn't count, building arks does'.

EDIT: an additional disadvantage to Long Bets is that they stash the stakes in a very low return fund (but one that should be next to invulnerable). Depending on your views about the future and your investment abilities, the opportunity cost could be substantial.