Three reasons off the top of my head:
- Windows RT isn't really an "edition" in the marketplace -- it's positioned more as an iOS type of operating system that happens to have the real Windows UI. Nobody would argue that iOS is an "edition" of Mac OS X, despite it being built on the same core.... right?
- For the most part, Microsoft doesn't use the word "Edition" in their product names anymore.
- To most people, "ARM" is a body part, not a CPU architecture.