My $0.02, this was mostly due to historical lagging a more out-of-the-box 'complete' solution like JSX/React, specifically in: (1) Poor native browser support

My $0.02, this was mostly due to historical lagging a more out-of-the-box 'complete' solution like JSX/React, specifically in: (1) Poor native browser support until recently, Firefox originally refused to support Web Components v0 at all (now called Custom Elements in v1, they are supported in all evergreen browsers mostly). (1a) Available polyfills were initially buggy and had severe performance issues on older browsers. (2) Lack of proper templating, though I personally quite adore template literals and approaches like e.g. `lit-html`. (3) Lack of robust styling API - custom styles felt coarse and tacked on in v0 especially, and were poorly supported by pre-processors like LESS/SASS. (4) Lots of impedance mismatch issues connecting standard JS/HTML/CSS libraries with the Shadow DOM.