Bitcoin is extremely popular, and therefore, it is the first cryptocurrency that people have bought and WILL buy. It is the original cryptocurrencies, the grandfather! The best thing is we can trade FIAT money directly for BTC, and that is not the case for most cryptocurrencies, except for ETH, Ripple, BCH and some others. If we want to buy other coins, then first we have to buy BTC, and it is not changing any time soon.