The problem is that if you're writing a Vue app, and you import a web component that was written in Preact, you're now loading both Vue and Preact onto your page. That means your app will take twice as long to load. That is a huge downside to weigh against the inconvenience of having to find a version of that widget that's built specifically for your framework, or even just building one yourself. The compatibility layer doesn't matter if nobody's going to use it.
> To be frank, I think in large part this is due to React
I'm not sure. I think React saw that the web components model was broken and didn't waste the effort trying to stick to it, and built their own separate React world that wasn't broken and that's why they've been so successful. It's a shame and an indictment of the web platform, but I think from their perspective it was the right decision.
The real shame would be if React's mindshare is the thing now holding us back from finally solving this at a platform level. I can only speculate about that, but it doesn't seem out of the question.