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?
If I'm not mistaken, Bitcoin is not the first cryptocurrency. But it is considered as the top cryptocurrency for its large market value, community, market cap and high market demand. Bitcoin is known for its best at long term investment. As its market value is increasing, the profit that its investors and users are earning will also increase. And I think, the profit is what makes Bitcoin more popular.