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?
The number 1 is the reason. You know about the FIRST in every field even if you are not entirely connected with it. So, same is the case with BTC, its the FIRST of many Crypto Currencies and when you are FIRST of something popular then you are the starter of any talk. Here, its not just alone that but even the value that makes BTC so popular.