The fact that newer versions of IE can't be run on older versions of Windows means that users using those older OS's have to choose another browser if they want to view pages with newer standards.
The newest version of Chrome according to Google support Windows XP SP2+. The newest version of IE can't even be run on Windows 7. Google has taken over some market share by supporting Microsoft OS's longer than Microsoft supports itself. I don't work for Microsoft so I don't know what their real goal is. I assume that they want users to pay to upgrade their OS so they can use the newest browser (or push them in that direction). Because of this, the people who don't want to upgrade their OS find current browsers that will support what they have. Unless MS wants to put IE's newest versions on older OS's they will lose market share on those platforms.
sorry but what do you mean by " The newest version of IE can't even be run on Windows 7. "
what version is that ? I am running a build of IE10 on windows 7 right now. it is a preview build and the final is coming. works fine for almost all sites I go to. the problem ones are the ones that for example refuse to see that this is IE10 and not IE 6 /7/8.