I don't understand why Microsoft divides its browsers this way. They seem to be the only one that actually has versions, and [security] updates within a version. Instead just have a single browser (instead of 5 or more counting from IE6) and any update increments the version. Both Firefox and Chrome work this way. They would have to allow installing the latest version of IE on any machine all the way back to XP, but…
I don't think they can. IE's (since IE5) is not your usual standalone browser application - it's an operating system component, that's used in a lot of places, starting right from the desktop. Tighter integration and mutual OS-browser dependence means it's not as easy as saying "ok, let's allow IE10 to run on Vista".
I do some work with various embedded extension APIs in different office and collaboration tools, and it's quite a mess trying to figure out what those web browsers actually are