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 think its mainly because it was the first. no other reason.
There are other alts out there that serve a much better purpose when it comes to operating as a currency, but yet BTC is still the most popular.
Nut i dont think it will always be this way.