Because market value of BTC is not at all stable or proven as the market is quite small. It is entirely imaginable that somebody will design similar, but slightly better system (faster transactions, more efficient implementation, whatever) that will cause most bitcoins traders to move there. Essentially, system like this depends entirely on market, while nothing has "inherent value", most physical things can be still exchanged for something (and actually gain value) after market crashes, when this kind of virtual market is abadoned, you are left with completely worthless big chunk of data.