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?
Because this is what started the all crypto, most of the people does not understand in crypto but they do know what is bitcoin, so most of what is popular is bitcoin.