I think because the inevitable result of that is that you constantly land on broken pages, just by following links, even if you start in some kind of enthusiast area that caters to such a browser.
You could mitigate that by only linking to pages that implement the HTML subset deemed valid. And instead of linking away to another domain, you could syndicate those pages on a centralized domain so to prevent loading dangerous or slow content on the open web. We could call it something like Accelerated Mobile Pages.