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 popular because many people are accepting and adapting to it. Many investors have turned into it so you could say it has already a strong foundation.