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 strongly agree with you, besides to dogecoin the ethereum is the another popular in the crypto world. I think the other reason why bitcoin is popular because of being legit and the big improvement in the price. And compare to the price, bitcoin is the only cryptocurrency that have an expensive value.
Value is not all, for the new investor, really value is the reason make many people thought Bitcoin is the best, is number one of cryptocurrency.
But when you research more knowledges about cryptocurrency, you will understand what thing make Bitcoin become interesting with the investor and many people in all social class.