I guess it comes down to preferences. Note polymer is not the same as the elements that come with it, it’s just the polyfills and extras on top of the current unfinished webcomponent specs.
I love using Polymer / the custom element library because it gives me security to have a maintained library of elements that work amazingly well across different screen sizes/devices. It’s most likely that for example the React Material UI components would not be maintained properly after a while, while all the components on the Polymer element catalog are officially maintained by Google.
I’ve never used react because it seemed awfully complicated versus Polymer. Why is it not becoming popular? Probably because until recently Polymer just wasn’t production ready. They just launched 1.0 about a month ago. Before that the polyfills were sort of working but they weren’t working 100% perfectly prior to 0.8. 0.5 still required a lot of testing and customization of your CSS code to make sure the styling of the elements was correct (behavior was great though).
I’d say it’s really a matter of building awesome sites and apps with Polymer now to show what you can do with it. I’m not sure about angular/react but Polymer still has quite a big footprint. The site I’m working on right now has about 500KB in size with all the custom elements and the polyfill included. Not exactly a big issue in countries with fast internet but in Asia it’s kind of a big deal. I’m spending lots of time making sure to get my Polymer footprint as low as possible right now for my products’ beta launch.