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 its mainly because it was the first. no other reason.
There are other alts out there that serve a much better purpose when it comes to operating as a currency, but yet BTC is still the most popular.
Nut i dont think it will always be this way.
In future bitcoin will more popular and every second person join bitcoin because they will better know bitcoin can settle their life than other income, and I think most of people leave their job for bitcoin because bitcoin has many way to earn money and that's why they will rich in very short time, so very soon government will also accept bitcoin by its popularity.