Since the supply of bitcoins grows so slowly, any increase in demand leads to higher prices.
That's known as deflation, and it's widely seen as a disaster when it happens to a real-world currency.
As money becomes more valuable, our incentive is to hold onto the money instead of spending it - slowing down the economy.
"What we want from a monetary system isn't to make people holding money rich; we want it to facilitate transactions and make the economy as a whole rich. And that's not at all what is happening in Bitcoin," Nobel Prize-winning economist Paul Krugman (seen here) wrote in 2011.