Avid has never been quick to qualify new OS releases and they do not publicly commit ahead of time when OSes will be officially supported.
Though hopefully they might provide some guidance here given folks seem to be having trouble getting things to work with Windows 8.
Anyhow the reason is for delays to support new OS releases is some combination of --
1. As Avid staff have explained here in the past there is a huge matrix of Avid and 3rd party interfaces, computer configuration, third party plugins, control surfaces, etc. they need to test against and this is just a huge effort.
2. Avid is not doing well, has laid off lots of people and reassigned others. Who know what affect his has on QA/testing and getting any product changes that may be necessary actually released. (I have no idea of any actual impact here).
3. Maybe Avid intends to tweek things in Pro Tools for Windows 8 UI features (I have no clue if this is planned).
4. Mac users have just gone through Lion and now Mountain Lion (and its not officially supported yet either). Since Mac is always more important than PCs the impact of doing that work may slow down official approval on Windows...
Take you choice, the answer will likely be some combination of the above.
But, and it the important BUT,... as is often the case for these "this is urgent dramas", there are obvious workable options should the worse case thing happen and Avid did not have a supported Pro Tools release on Windows 8. There is absolutely nothing forcing you to buy/use a Windows 8 PC. Windows 7 is fully supported and OEMs will likely still sell them fro some time, and you'll have the even better option of buying a Windows 8 Pro PC and using the downgrade license ability to run Windows 7.
Darryl