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.
It might be the features of bitcoin that is introduced in the market on 2009 its such a game changer in the old industry where medium of exchange is being decentralized where there are major difference with having your money in banks,
also bitcoin is so popular because its the mother of all crypto its the first one to be introduced in the market so people will really knew bitcoin over other cryptos in the market