People are holding bitcoin as there is certainly a future to it. Why would you hold any other investments such as bonds, gold, or stocks? Because you believe in them. Believe that the value will go higher and so will the margin of profit. Bitcoin is built entirely upon this network of user trust. Ergo, the more people believe in it - the more will hold into it and the more the value will go up. We have seen bitcoins value dip a bit here and there, but it has always ended up at a much higher value a few weeks, maybe months later. That is the main point of holding and that is why more and more people hold on to it
That's what had been driving the Bitcoin price higher and higher all these years
But you don't specify why exactly all these folks believed in Bitcoin and what specifically they believed in. But I can clarify it for you. People believed in Bitcoin simply because it was giving them profits. In other words, they believed in its future growth. Obviously, if Bitcoin didn't in fact show this growth, people would soon lose their faith. That was kinda a self-sustaining and self-feeding loop. And now it is about time this faith to be tested for real. Mark my words, if Bitcoin stagnates, people will quickly start turning their backs to it. It is no longer 2013, it is already 2017 where there are quite a few really competitive coins available, and no one will blindly believing if that means losing profits