It has very limited use as a serious transaction system while there is so much volatility. Prices are therefore pegged to local currencies, and if you want to keep that then it's no longer low cost because conversion to/from local currency costs just as much. The speculation and (eventually) deflationary nature of BTC significantly hinders the chances of it ever stabilising, and therefore a true 'bitcoin economy' emerging (ie. where people do not need to peg to or convert to local currencies).
Then there is the fact that it is not a limited good-- someone could quite easily come along with a BetterCoin making BTC almost useless-- there are various things that could be improved, eg privacy, or the slow confirmation times.
E: also, there will be an 'oversupply'-- Whoever buys these bitcoins won't be buying other BTC because of it -- it's irrational to think that the auction is creating a significant new demand that wouldn't have existed otherwise! Selling over weeks/months would still affect the price, although more in the normal supply/demand manner than causing a crash/panic. But that could be happening before any of these BTC are sold anyway!