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 more popular than other coins. Bitcoin is the first coin to be created, so bitcoin is believed to be the strongest coin and the first choice in investment, but it is true, bitcoin has proven its strength in previous years and will be more popular in the future.