I'm not sure whether "it" in Rasmus's second paragraph is referring specifically to the fact that you can submit old predictions, or to the idea of the site as a whole; but the possibility -- nay, the certainty -- of considerable selection bias makes this (to me) not at all like a database of all pundit predictions, but more another form of entertainment.
Don't misunderstand me; I think it's an excellent form of entertainment, and entertainment with an important serious side. But even if someone is represented by a dozen predictions on Wrong Tomorrow, all of them (correctly) marked WRONG, that could just mean that it's only the wackiest 1% of their predictions that have been submitted. Which would show that they're far from infallible, but that's hardly news.
Quite possibly this is the best one can do without a large paid staff (which introduces troubles aplenty of its own); it's just not feasible to track every single testable prediction made by any pundit, and if that started being done and noticed the likely result is that pundits would start taking more care to make their predictions untestable.
Wrong Tomorrow by Maciej Cegłowski is a very simple site for listing pundit predictions and tracking them [FAQ]. It doesn't come with prices and active betting... but a simple registry of this kind can scale much faster than a market, and right now we're in a situation where no one is bothering to track pundit predictions or report on pundit track records. Predictions are produced as simple entertainment or as simple political theater, without the slightest fear of accountability.
This site is missing some features, but it looks to me like a starting attempt at what's needed - a Wikipedia-like, user-contributed, low-barrier-to-entry database of all pundit predictions, past and present.