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?
Bitcoin is the most popular cryptocurrency. Because Bitcoin has been able to prove to all its users that Bitcoin can make someone a millionaire. And because Bitcoin is one of the most promising coins so there are so many who believe in this coin.