On a (slightly) more serious note I'm with Morthoron on this as well.
If it needed to be explained internally then maybe (!) something like: the loss of the One somehow scarred the fea, so much so that the fea rebuilding its hroa could not overcome this 'spiritual scar' and thus Sauron was forced to reflect this in the body.
Or something like that but better
But again I think it was just too 'poetic' or 'right' (for JRRT) on an external level -- just like I think Tolkien knew that Sauron must take the Ring to Numenor (he couldn't leave it behind), and so Sauron's fea 'must' have somehow carried the physical One back to Middle-earth, his spirit being so tied to the One.
Or something.