I believe that they're popular mainly because well it was the first of all cryptocurrencies to be made. Honestly most of the other coins are scams and greedy ways that the developers make money. The only other coin I can think of that isn't a scam are dogecoins! What do you think?
The technology was introduced first by bitcoin. So much so that the word cryptocurrency oftentimes means bitcoin. Thencryptpcurrency industry owe bitcoin it's current fame and popularity. Without bitcoin I doubt the cryptocurrency world would even exist. Because of bitcoin the cryptocurrency world becamendiverse as altcoins start popping out of nowhere. And ICOs started to produce their own cryptocurrency.