Two reasons: slow transactions and high fees.
Bitcoin has the first mover advantage, but I don't think it will be on top of the Crypto world forever.
Like we saw with the rise of the Internet, "top projects" of today (AOL, Yahoo, etc.) will fall by the wayside as new, more efficient, and more useful projects (Google, Amazon, etc.) enter the space and gain recognition. Improvement is the name of the game. Who can come along and add the most value to the crypto community?
Cryptocurrency is still in its very early stages, and ultimately, this race is a marathon and not a sprint. I have my ideas, but I am curious about yours. Who will we still be talking about in 10, 15, 20 years?. . .and which projects are likely to fade due to their lack of practical use cases?