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 only coin that you think is not a scam is not even on the top 10 or 20 or 30 on the list. So you better study the other top coins out there because there are definitely more than just one legit altcoin out there. Bitcoin is the most trusted because it is the prime-mover so to speak. Bitcoin started everything. Bitcoin is the father of all cryptocurrencies. It has earned the trust that people hardly gave to something that is radical as much as strange. Why? Because it offered something really new yet helpful.