The reason why Bitcoin is the number one cryptocurrency is pretty simple: It was the first one. This first mover advantage persists, because Bitcoin quickly established the biggest support both from users and from developers. Having the biggest user and developer base maintains a strong network effect, that tends to draw in more users and developers, because the system appears attractive and is well-established as a payment system.

For any altcoin to compete with Bitcoin, it would have to offer something revolutionary new and useful, that can't be implemented by Bitcoin. Today's altcoins are mainly a testing ground for new ideas, that will end up being added to Bitcoin if they are good, leaving the altcoins that first introduced them behind (a current example for could be rootstock vs. ethereum).

ya.ya.yo!