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?
I agree with you, the first reason because bitcoin is the first cryptocurrencies created. Apart from that the value of bitcoin is fantastic. Its makes people interested to know bitcoin. The trust, strength and security of bitcoin keep people talking about it.