- Web Components as a technology is not a competitor for React or Angular – it is an updated version of DOM with great incapsulation.
- Web Components are not very popular because of two main reasons: 1) no server-side rendering 2) no full browser support, which is matter of months or even years.
The main problem, actually — impossibility of writing 100% compatible polyfills for ssr or old browser, because you cannot reach the same level of incapsulation (the main point of WC) with old DOM. So, WC cannot be adopted as fast as es6/css3/html5.