It may be possible to back-port the OpenGL runtime (when we make it) to XP SP1 (or even no service pack at all!), but it gets increasingly difficult the further back you go. For example, C2 uses a local HTTP server to preview HTML5 games on. The HTTP server is provided by XP SP2 and later. Therefore, to support SP1 and earlier, we'd have to find an alternative HTTP server API or write one ourselves, which would cost time/money, and hold back other features we really need to do. Then, there might be 3 or 4 other features we need to repeat that process for, because they're only supported by SP2 and up. So it could be a lot of work, all for people who haven't installed a free update that was released about 7 years ago.
So writing a runtime from scratch we might be able to do it, but from the developer's perspective, it makes things more complicated and time-consuming, which is especially expensive for small businesses that don't have whole teams of programmers to throw at problems.