Maybe it is because all the major browsers, except for chrome, don't support them. I've build this, because the polymer project made web development exciting for me :) but javascript didn't. And go is awesoooome!
Seems like the standardization effort has stalled out. Did React kill the perceived need for it?
Web components are a bit low-level, so you have libraries and frameworks like Polymer, X-Tag, Svelte, SkateJS building on top of the standard to make the development fun and friendly.