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 it is the first cryptocurrency that was created. It is also the most popular because the people nowadays are using bitcoins to pay their bills and their expenses.