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, the main thing is that bitcoin is the god father of all crypto-currencies in the world. And secondly it has a lot number of trusted users and traders and transaction is 90% safe in the bitcoin platform. That's why people around the world are satisfied with bitcoin. More the satisfaction, more the popularity is.
