How come it's not correcting itself?

Sam Wang writes about the deficiencies of Intrade prediction market:

Even when lots of data are available, such as political polls,  InTrade can still fail. One simple reason is bias: InTrade bettors appear to skew Republican. This could explain why there is such a mismatch between the poll-based Obama win probability (>99% for an election today, probably >80% in November) and the InTrade price (equivalent to a probability of about 0.56). This could be excused on the grounds that the election is far off, and there is uncertainty as to what will happen in the next 4 months. However, there is a third flaw. As I’ve written before, InTrade bettors are habitually underconfident in the face of polling data, even on the eve of an election. Even a 10-point lead in a race is insufficient to drive a market-based probability estimate above 80%. This is perplexing since such a lead is basically a sure thing.

A functional market should be self-correcting. Any systematic bias such as underconfidence or leaning Republican creates an arbitrage opportunity which should be expected to draw new bettors until the point when prices adjust and bias disappears. The question is why this isn't happening in this case.