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.
It also prejudices Microsoft to what basically is an implementation detail. Windows RT has lots of restrictions (like Microsoft signing of drivers, no desktop apps etc) which are a consequence of Microsoft's policies, not a consequence of the architecture.
Declaring Windows-RT to be "Windows on ARM" makes it hard for Microsoft to later blend the two by allowing you to run Windows8 (i.e. not RT) on ARM desktops or allowing people to run RT on x86 tablets.
That all being said, I think "Windows8 Tablet" would have been a much better name than "Windows RT".