In short: Although Bitcoin may have some admirable features as a medium of exchange, why do you suppose that it can represent a popularly desirable store of value when there are so many other excellent alternatives?
Note that a successful currency requires both features. Absent a reason to store value in bitcoins, people will keep their stock of wealth in other assets and then exchange them into bitcoins only when they need to engage in a transaction. But in that case, the real value of a bitcoin will be approximately zero; it will be slightly positive if the market deems its virtues as a medium of exchange to exceed those of other currencies, but beyond that, it will be zero.
Possessions increase to fill the space available for their storage. -- Ryan