Because bitcoin is the first crypto and becomes the mother of the other crypto. Bitcoin did his best from the beginning to be what it is today.
It's not the main reason why bitcoin is the most popular cryptocurrency. Even if it's the first cryptocurrency, there are still a lot of potential altcoins. As for me, the main reason why bitcoin is the most popular cryptocurrency is because of its volatility. Bitcoin price is not stable, it may pump or dump that it gains more investors to invest in bitcoin. Lastly, you'll be able to earn huge profit in a short period of time. Because of that, bitcoin gains popularity every now and then.