• It doesn't have Facebook backing it
• It isn't as easily implementable in your app without changing a lot of things
• Examples look more complex at first sight. Doesn't have jsx (which is really more of an obfuscation than a helper, but hey, it really separates cleanly logic and display)
• It doesn't have Facebook backing it

It is easily explainable.

Nobody's going to replace React anytime soon. To do that, you have to be a lot better. Not just a little. (Shamelessly copied and manipulated from here)

• Find a much simpler mind model for solving web application issues
  • React teached us to forget about Object.observe and it told us how well suited an unidirectional data flow is.
• Be much faster at rendering times
  • 100x faster isn't fast enough because React is already fast.
• Have a much shorter compile/transpile time
  • Developer experience matters. Compile, hot-reload or hot-replace in less than a blink of an eye.
• Help targeting all relevant platforms
  • React supports Browsers, iOS, Android, OpenGL, soon'ish Windows Platforms and Tizen and TV's. Thanks to text rendering it supports virtually any platform
• Have more daily active users to prove it's production readiness
  • You have to beat Facebook first!