That's because Metro is supposed to run lean as a tablet experience so they don't want every Tom, Dick and Harry just wrapping their windows apps to run on it.

That's because Metro is supposed to run lean as a tablet experience so they don't want every Tom, Dick and Harry just wrapping their windows apps to run on it. Chrome/Firefox/etc. already have Windows apps so now they have to create an app for Metro. No different if they were to create a Firefox/Chrome for Iphone vs MacOS.