"I see no good reason why they refuse to do svg in text/html mode"

This probably breaks major assumptions inside the browser engines. Rendering most image types is a one-and-done type deal, just setting up a few calls to libjpeg or libpng, but SVG actually has to be XML-parsed and rendered element by element on page load. To do it in text/html, you'd need to hook together the html parser with the XML parser with the image generator. That's a lot of work to break the rules!