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?
Well if you'll think of it that ay, bitcoin is popular because it's been the oldest of all crypto coin so it is only normal that most community have already known it which makes it that popular which unlike other coin that has only been released like a year or two. There is ETH which is showing potential to be popular yet it is not a coin.
Bitcoin provides a lot of opportunity to both young and adults in the community so it is only normal for people to spread word about something where they can all benefit.
Maybe because it is the first cryptocurrency? And as you can see the volume of bitcoin it is really high than any other when compared to other cryptocurrency. And why is that happen? Because maybe many investors trusting bitcoin than any other crypto.