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.
Being the first ever bitcoin the sets a name and a value a market bitcoin is the most preferable cryto-currency to used. And as the days goes by bitcoin can adopt the changes and developments of the technology brought by the inevitable power of progress meaning to say that bitcoin was the developing each day on its transaction, value , price and security. Making bitcoin as the number one priority or choice of people to use on a mode of transaction through the web as crypto. And also the support given by people can help on bitcoin decentralization and development.