People bought, exactly the same number of coins. All in all, people traded, and they were willing to trade at lower and lower prices.
To answer the OP question, here is another question: why did the price triple over only two weeks? It's all completely irrational speculative gamble. The fundamentals have not changed before, during, and after the bubble. Just like in 2011, this will shake out get-rich-fast ignoramuses, and Bitcoin will finally have time again to grow, develop business infrastructure, etc. - without dramas like trade lags, backlogs of unverified new accounts, hacks...
+1
This is another event needed by
BTC. Fling out the get rich quick schemers (who likely trade emotionally) while learning the weak points (MtGox, DDOS, getting Fiat into an exchange, exchange malfeasance, etc).
Now we can continue to create the scaffolding that
MAY support a real economy of
BTC users in the future. If
BTC could have an associated human age (anthropomorphic metaphors are easy) it would be anywhere from 4-9 years old. There is a LOT of maturing yet to happen for users, exchanges, economy, etc.
Luckily, the blockchain will survive all of our mistakes and learning curves.