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?
Because Bitcoin is the first digital currency. Bitcoin is the KING of all cryptocurrency. As you can see that Bitcoin is the top 1 in the coinmarketcap and Bitcoin has the very high total market cap compare to other cryptocurrency. But now, Bitcoin is already have high transaction fees. I don't want to to use anymore.