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 first created cryptocurrency and has been proven to be sustainable since then even after facing a lot of fud, bitcoin still exist. That's why people confidence with bitcoin rather than the other cryptocurrencies and also bitcoin itself already has a strong brand image.