bitcoin is the king because of a high price and also how the popularity of bitcoin is popular with many people, so I think it's a natural thing for bitcoin if bitcoin becomes king.
Bitcoin has high price and it is famous because it was the first crypto currency which was created and introduced to the world. For many, Crypto-currency and Bitcoin are interchangeable terms.
It is far more liquid and called as King of crypto because it can be bought and sold at almost 100% of exchanges which are in operation at the time and we need to buy bitcoin first and then convert it to other coins because fiat to altcoin trading pairs are really very few and more than 80% of tokens/altcoins are bought with btc pair only (along with some with eth too).