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?
Actually you'd be surprised at the number of people thinking that altcoins are superior to bitcoin.
However as you said, the majority still prefer bitcoin as their day to day cryptocurrency and their go to place to store their wealth. I think that the main reason is bitcoin is the first cryptocurrency, and the longest lasting.
Another reason is that altcoins have proven to die off over time. For instance, Namecoin was the first altcoin but now nobody uses it. It used to have a lot of hype as well. Litecoin was ASIC resistant but that wore off over time somewhat, so now its popularity is not even half of Ethereum's. Based on this we can safely deduce that Ethereum and other altcoins will probably be replaced by new altcoins in 4-5 years time.
The fact is that you can always have better lines of code based on the previous coin, but you can't write the reputation of a coin on lines of code. And bitcoin has a lot of reputation for what it does.
Bitcoin is the first and original cryptocurrency. What made it popular is because of its value. It has the highest value rather than other currencies. Because of its value, many newbie users are more getting interested to use it. Nowadays, the value of bitcoin is soaring high. Many investors and traders has a big trust in bitcoin because it is more reliable, secure and safe to use.
