It's quite simple. Bitcoin has the first mover advantage and it's also the most widely used one. Whenever anyone wants to get involved with the cryptoshere (blockchains, cryptocurrencies, decentralized technologies etc) Bitcoin is the first thing they look into before proceeding. The problem with altcoins lies in the fact that they pose most risks and majority of them lack a major use case – there's at least over 100s of altcoins whose only purpose is to facilitate peer to peer transfer. There's a dozens of others offering almost the same functionalities/use.

By the way, Altcoins never failed, they're right where they are – providing supports in areas where bitcoin falls short.