https://hacks.mozilla.org/2015/06/the-state-of-web-component... (ctrl+f "HTML Imports")
"HTML Imports do lend themselves well to a simpler/more declarative workflow. [...] Imports don’t offer enough control to be taken seriously as a dependency management solution. [...] Mozilla had a working implementation behind a flag, but struggled through an incomplete specification."
which to me reads a bit like: "js frameworks won't use it, and the spec didn't get enough love because everybody's only thinking about frameworks rather than vanilla-js."