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.
i think that may not be the reason, being second may not be the only reason , but i think bitcoin is more trusted than any other currency which is making bitcoin more popular and trusted. in fact the popularity of bitcoin is increasing continously which is introducing bitcoin in all over the world and therefore the popularity of bitcoin is increase day by day as compare to any other currecy, and hope that very soon bitcoin will be the most popular currncy of the world.