If a new version of, say, Chrome were to drop the Mozilla/5.0 from its UA and break a website because it relied on Mozilla/* in its UA detection (there are STILL sites that do this), who would users blame? Chrome, obviously.
If you're trying to make a new web browser, it's even worse--people won't use it if it breaks the sites that use it. And the web developer would say "it works in all the major browsers, what's wrong with it and why should I spend the time to fix it for your shitty new browser?"
The problem is that the blame for broken sites is universally attributed to browsers, not website developers.
