I think it's easier to just come up with a new number than a new name. Even Microsoft has returned to their regular numbering scheme again, going from Windows 2, 3, 3.1, 3.11 to 95, 98, ME, 2000, XP, Vista and now back to 7.
Besides, using letter codes might result in unwanted associations. E.g. we have Windows CE, ME and NT which are three different Windows system that were all operational at about the same moment. (Just put the letters together.) At least, with numbers you don't have the risk of accidently spelling out some strange words. (Then again, Borland/Codegear/Embarcadero did skip version 13 of the Delphi RAD studio.) People also tend to avoid version numbers like 6.66 or 6.6.6 for some devilish reasons...