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?
Of course the main reason here is because bitcoin is the first cuptocurrency to be introduced as far as I know. It was the first to be known and to be popularized. Another thing is, bitcoin has become ao volatile that it makes it perfect for trading. It is best for holding too. We can say that bitcoin is the main cryptocurrency because almost all the cryptocurrencies out there deped on bitcoin and its price.