Well, let's flip the question around -- why are there three major open-source rendering engines in the world right now already? Why did Google fork Blink from WebKit?

The answer seems to be, major browser vendors have a hard time cooperating at the engine level. Apple and Google had different opinions over WebKit, which eventually led to it being forked. Microsoft has no real reason to think that it could partner with Apple or Google any more successfully than they partnered with each other in developing a rendering engine.