Re: Internet Explorer 9 tops first HTML5 conformance test
The reason why they need a crossplatform browser is because web application are becoming more the thing to use. Microsoft only put their eggs in one basket(as the saying goes) because they have a monopoly on the market, but in regard to browser share, that's being eaten away.
As new platforms get popular, you need to make software for them, just like Google and Mozilla have done. All Microsoft can do is gain back Windows users, rather than an extra 10% plus the other platforms have and that's not even including the mobile market.