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?
Yes bitcoin is the most popular brcause it has an exchanger in all country that using it and you can pay bitcoin in online shops pay bills and remittances.