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 that bitcoin is more popular than other coins because bitcoin is the first of all cryptoes made. But I do not agree with your opinion that the only other coin that is not a scam is dogecoin, now many promising growth coins like minex, oyster, and bitboot.