Is that the stakes weren't high enough, or is the whole idea flawed?:
If obese individuals have time-inconsistent preferences then commitment mechanisms, such as personal gambles, should help them restrain their short-term impulses and lose weight. Correspondence with the bettors confirms that this is their primary motivation. However, it appears that the bettors in our sample are not particularly skilled at choosing effective commitment mechanisms. Despite payoffs of as high as $7350, approximately 80% of people who spend money to bet on their own behaviour end up losing their bets.
That's from Nicholas Burger and John Lynham. Here is further information, from Economics Letters. A gated copy is here. A related paper, with similar results, is here. The wise Alex, on same topic, is here.