Because devs will never agree on what should be the only one true JS Framework. Like programmers will never agree on what the one true language should look like. Just take JS itself, some devs think it's the "best language ever", others despise it.
Obviously things like unidirectional data flow is getting traction in every big framework. Yet some people think it's "magics" since the DOM doesn't work that way. Who got it wrong? all these fancy framework creators or people that wrote the DOM ? well the point is both were created for different needs , thus the reason why there will never be one true framework. Because developers have different needs and project requirements.
( https://xkcd.com/927/ )
I don't know why anyone would want One True JS framework. Many of them provide different things. Angular has the kitchen-sink approach handling everything from templating, model-binding, routing and ajax requests and on the other end of the spectrum there are small libraries that do one thing well and encourage their users to adopt other libraries to do other stuff. Choice is a good thing.
Everyone is converging to components based development and web-components are a standard, so they will be more similar than with the "we do our own idea of MVC" in the past.
the hardest part of making a framework isn't manpower; it's design, architecture, algorithms...
Just use React as the other two are copying/converging toward React.
All the other web app UI frameworks? Noise, they won't amount to much and add very little. The last release worth paying attention to was React 2 years ago. Hardly a breakneck pace.
(Though having tested React recently, its the best of the ones I have used).