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?
People have the tendency to trust and believe the original, the pioneer, or the root. Since bitcoin started this technology of digital currency, other altcoin in the market is somehow connected on bitcoin in terms of progress and demand. There is a particular point where popularity is related to where it all begun, specially if it become successful at the first place.