We can’t use the carrot of new HTML5 features to attract users, since authors won’t use them widely until they are widely supported in the install base, and users won’t upgrade if the new browser has a worse experience with existing pages than their old browsers. …

We only have as much power as the browser vendors and the Web designers grant us by deigning to listen to us. If we say something they don’t like, they ignore us, and we become powerless. In some areas, we have a lot of leeway, because browser vendors may not care about the exact details of a solution. But if we stray outside their constraints—e.g. if we require something that would change the rendering of many existing pages—then they’ll ignore us.

— Ian Hickson (emphasis mine)