It would seem to me that the most important reason for the delay is that it puts the focus solely on developing the Metro apps and building out their ecosystem. Considering that the RT tablets simply don't use the desktop interface for programs other than Microsoft Office, it would make most sense (I believe) for Microsoft to put the emphasis on building their app ecosystem first, because you'll always have x86 programs being developed later on. With that in mind, it makes the Metro interface more justifiable, promotes Microsoft's moderation/streamlining of it's content ecosystem to get your programs (and once more poeple are comfortable with that, then the rest of the media as well, such as music and videos), and makes it more competitive to the other tablet ecosystems at a faster pace than it would to simply have both tablet versions available at once.
Unfortunately, it takes away from the potential users who would prefer to have the x86 version of the Surface right away, but I think that's the main reason why, if I had to guess.