Like somebody mentioned earlier, the selloff was caused by greed.  Short-term speculation and the desire to make a quick buck.  It doesn't just go one way - greed isn't something that only exists when the price is going up, and it isn't gone when the price is stable.

The fact that bitcoin has not completely disintegrated and seems to have landed somewhere between $70 and $90 is a clear sign that it's not going anywhere.  Investing at this point is simply an acknowledgment of that.  The bust scared a lot of speculators; what remains is a growing number of people who believe in bitcoin's long-term potential.