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?
Yes i agree that because bitcoin is the firs its the main reason why it is popular. but there are other factor why bitcoin is famous. its easy to use faster and reliable. but today i dont think the popularity of bitcoin is still remain because of lots of alternative coins are pumping from some where and has a potential to over run bitcoin in terms of popularity,.