Metcalfe's law says that network grow proportionally to the number of users https://en.wikipedia.org/wiki/Metcalfe%27s_law. That is why google plus never caught on with facebook, every user was already on facebook. In the case of Bitcoin, this goes stronger, it is easier to jump from a social network to another. In case to go from BTC to alts it has a monetary cost so the jump is a lot harder. Also, BTC does not have a pre-mine, and Satoshi has never moved his stash from mining, giving more confidence.