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 think bitcoin is the most popular cryptocurrency because there are many people saying it is the strongest currency among others. Whatever coin you are holding now, bitcoin is the last destination of it. I mean bitcoin is the last currency when you are going to trade different coins you have. Bitcoin cannot replace other currency.