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?
Because as you said everything started with bitcoin. All other cryptos were inspired by bitcoin. So it will be the most popular cryptocurrency bitcoin for a long time.