why not? 
Because there are too many rich people in the world with more fiat money than you can ever dream of who are itching to spend it on something before it becomes worthless, yet afraid to spend it at the same time for fear of it causing inflationary effects on the economy and destroying even more of debt-based assets they also own in large quantities.
not big reason to me,
seems like you thinking everybody is stupid around...
Then how about the accellerating aggregate demand for bitcoin as a result of broadening adoption and increasing coin distribution? Or the number of hardware devices being specifically engineered in a capital intensive fashion to increase bitcoin mining efficiency specifically? Or what about the network hashrate & the first-mover advantage of increasing awareness of bitcoin in mass media causing too large of a barrier to entry for competing alt-coins that provide marginal to zero improvement over the blockchain protocol. Or more simply, the fact that most people can only learn about and properly understand one crypto at a time. This will naturally draw more people towards the dominant entity, which is bitcoin.
For the moment at least. Things may change if a new altcoin improves signifigantly over the bitcoin protocol, or achieves dramaticly more 'fame' than bitcoin already has. But once the price starts heading north of 10,000 you might perhaps get people looking more closely at which altcoins are here for the 'long-term' and which are flash-in-the pan pump & dump schemes.