So far, bitcoin has had little competition from other crypto-currencies. This may change in the future, considering that
now it's worth a lot of money to have a successful crypto. Apple may launch one, Google may launch one, both could have cute mascots.
Bitcoin is just the first and a good one, but not perfect. And against the marketing power of Apple or Google, bitcoin would eventually lose.
Didn't answer my question, but ok. Maybe your answer should have been "because if bitcoin no longer appreciates in value, it would in that case no longer be a speculative vehicle, so all the speculators would sell their bitcoins and leave (to another alt coin)." But by the time bitcoin becomes THAT stable with very little gains (or loses), all the speculators would have already left.