One of the reasons that React, Vue, and other reactive frameworks have become so prevalent is because of Virtual DOM reducing the number of Physical DOM updates

One of the reasons that React, Vue, and other reactive frameworks have become so prevalent is because of Virtual DOM reducing the number of Physical DOM updates. Web components are a nice standard, but don't force reactive DOM updates. Web components can contain their own script block and can do whatever updates they want, breaking reactive patterns in apps. In order to keep a reactive app behaving properly, developers will have to vet web components to ensure they work properly with their reactive app. In all likelihood everyone will continue re-implementing components in the reactive framework their app relies on.