Microsoft seems to be tied to the Windows brand when it is not appropriate and even harmful to the prospects of a product. Would you buy a Microsoft Windows Xbox?
Windows RT brought to mind all the negatives of the Windows brand: viruses, instability, insecurity.
Yet the Windows RT name here, as DigitAl56K noted above, did not include the brand positives: Familiar UI, existing software and games.
Coming up with a new product name is difficult, especially for a global company. Using the existing brand plus two random-to-consumers letters was a wimp-out that added nothing to differentiate this radical departure from the rest of the Windows brand.