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?
Care to enlighten why dogecoins?
I think BTC was introduced as one of the way to go cashless. Virtual payment in short and BTC is the first virtual currency. More merchants used BTC as payment gateway too thus the popularity. I can understand the needs of having ETH, Golem, Sia, etc because they all serve a different purpose from BTC but what I can't really comprehend are coins that have the same purpose as BTC. Sure it's always good to have competition and LTC has the same purpose but they are trying to be better than BTC by having faster transaction speed, etc. Zcash also has the same purpose as BTC but they tried to differentiate themself as a more secure coin. What about Monero, Dash, etc??