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?
Of all the coins out there the only one you think isn't a scam is bitcoin and doge? Crazy! I think you're confusing a scam with simply being overvalued.
Also to answer the topic, it's because it was first on the scene. Plenty of other cryptos are technically "better" than Bitcoin, but they'll never make it, they're simply too late.