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?
I think because bitcoin is the first crypto coin, so it makes sense if bitcoin is the most popular coin. Indeed many altcoins are created, but iku will not be able to beat the popularity of bitcoin.