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?
Yes, the biggest influence on bitcoin is that it is the oldest cryptocurrency and it has gained a lot of confidence. But cryptocurrency like ethereum, has also gained an important spot in this business. This is why investment projects like propthereum.io use it!