The one basic issue is that the value of bitcoin is not "pinned" to anything obvious. For example, if you buy Apple stock, that is connected to the well being of the Apple corporation. With bitcoin, it's not immediately obvious what influences the price of bitcoin, which makes it less possible to figure out what can change prices.
The other issue is that bitcoin is still quite new, and there is still a lot of room for "unknown unknowns" and "growing pains". We've been trading stocks for hundreds of years, so a lot of the failure modes are well known. With bitcoin, we don't know what the failure modes are.
I love the term unknown unknowns which basically means we don't even know what we don't know. In a way, the Apple example and bitcoin example can be the same. They both depend on the buy demand. If people are buying the product, then it's a success. Otherwise, it's going to fail eventually. But one huge difference is that there can be someone playing with the bitcoin price. He can be holding thousands of bitcoins and just sell them for no reason. I think that's what he was talking about "it's not immediately obvious what makes the price of bitcoin move."