I suspect that it is two reasons they don't have IE9 on XP:
1) They don't want people using XP. If other browser vendors want to take it on, I think MS has decided that they're fine with that. But they don't want to enable XP anymore.
2) The test matrix is a huge test cost. Doing XP means cutting something else. I think they decided that the cost of supporting XP wasn't worth it when they looked at how it effected their roadmap.
Comments
I suspect that it is two reasons they don't have IE9 on XP:
1) They don't want people using XP. If other browser vendors want to take it on, I think MS has decided that they're fine with that. But they don't want to enable XP anymore.
2) The test matrix is a huge test cost. Doing XP means cutting something else. I think they decided that the cost of supporting XP wasn't worth it when they looked at how it effected their roadmap.