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.
So if this is an unstable, unregulated market that shouldn't be used for anything more serious than pizza at what point is that going to change? What magical thing is going effect to change?
And if that thing doesn't exist, what is this apart from a more complicated way to waste electricity than Seti@Home?
The assumption/hope is that it will stabilize as it grows, but this will take time. It's not gonna be a magical thing/moment.
It only has value if people accept it as payment. Cryptography is only part of it.