Bitcoin Holds this dominance for the only reason that Internet, the Media and your entire entourage heard and talks more about BITCOIN itself rather than the entire Crypto-Currency Market. Those people are not aware of how big this Market is, and don't understand how the next Amazon, Youtube, Businesses are being built on this same decentralised Market.
PS: I'm pretty sure half the people investing in BTC (btc only) don't even know how it works, or what decentralizing a currency means. Asking them to get into Exchange Platforms, Alt-coins, ICOs is probably too far ahead. All of us took time to get it, and not everyone thinks this is time worth investing into "concepts".