Because it being the first successful implementation...it has 9 years of history at this point. It has proven itself resilient to hacks, ddos, Chinese bans, threats of regulation, exchange scandals, and bad press, and yet the protocol just keeps humming along. It being the first, all other cryptos are traded in terms of BTC. The network effect, and the mind share of BTC are substantial at this point. BTC has one largely fixable problem: it doesn't scale. Lightning network tests prove that it can if a second layer is added. Once this rolls out, the game will change.