But why ? Because it's the first cryptocurrency created, and the most used in the market ? What makes you believe that Bitcoin will be the eternal king of the market, and something else won't replace it ?
I think the main reason is because its the most trusted cryptocurrency in the market at this moment that is why many are believing in Bitcoin. I'm not saying that it will be the eternal king because that are the most exaggerated words you can use in the market. As most altcoins are based on Bitcoin's price, what I can say is it will be on the top of the market in terms of market cap but who knows? Somebody can top Bitcoin in the future. 
Anything is possible on the market.