Barring outside factors (hacking, breach of trust, etc) the price of bitcoin is naturally bullish given the current ways to trade them. Since there is no large scale operation providing a way to short bitcoins, there is no one "investing" in their price decline. This simply means that anyone buying in using USD now needs to cash out at a higher price to make a profit (on most exchanges) and anyone who has coins would not want to make the market tank since that just devalues what they have.
I realize the above is a simplification and there are many other factors (and there could be malicious participants), but overall this will continue to push the price up.
I realize the above is a simplification and there are many other factors (and there could be malicious participants), but overall this will continue to push the price up.