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?
Bitcoin is considered to be the most popular because it led the whole crypto world to expand. It is the first crypto and the fastest growing currency in the history. Many people support it and it is still growing. It's core purpose at first is to make a currency for the internet but the hype exploded later on when the price starts to grow.