Because new currency can be created. If someone spends a couple million to buy currency like what happened with bitcoin in June of 2011, rather than early adopter "quantitative easing" e.g. a bunch of old, cheap coins being sold on the market bringing the price down, people will create new currency until the price levels out to somewhere around the cost to produce plus a profit margin. Anyone attempting to manipulate the market upwards is guaranteed to lose money. Anyone attempting to manipulate the market downwards gives the opportunity for arbitrage whereby people will buy currency because it currently costs less than what it cost to produce. Energy, computer hardware, and time [to create money] define the value rather than a closed and limited commodity. There's no way to hoard all the coins. People will just make more and you will look foolish.
One of the great things with a system like this is that you don't need an exchange to determine the value. Anyone with access to a reasonable computer could determine how much effort including hardware, electricity, and time it would take to create X amount of currency and then realize that exchanging 2 coins for a loaf of bread is a reasonable value, for example. The tricky part was making it so that coins don't lose value due to efficiency gains in hardware, or if they do, that existing account balances are increased to make up for it. Thus part of the reason behind giving lots of coins away.