It's usually because it's tied to some sort of OS-level new feature. In the case of IE10, it's the rendering engine for (some kinds of) Metro style apps. If you're providing infrastructure for some new features, it's easy to rely on other new infrastructure, and pretty soon you can only run on the new OS. It probably was a serious effort to back-port it like this, although it's also possible IE10 represents a new model that changes things.