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?
You are right for sure! People who don't care about cryptocurrency and financial makrets at all could hear about bitcoin from somewhere accidently. These people will not care about cryptocurrency in future, but they already know a little bit about BTC. In mass media we are not being informed about altcoins. I personally paid attention on BTC only when I visited crypto exchange for the first time in 2016. I heard about BTC, but had no any info that there are another cryptoassets. Thats why BTC is #1.