I wasn't being clear (I'm not sure how, but apparently not...)- those two piano sonatas whose first movements and last movements are in different keys are from quite early in Schubert's career (D279 and another I think?) - and that they are unfinished is an inference, not a known fact. It's been several places and times stated that they are unfinished, but usually, I think, on the assumption that he would have completed them with a finale in the same key as their opening movements, not because we know him to have ever intended to return to them based on letters of his, etc.- at least, not that anyone's ever told me. (Likewise, I think, the Handel concerto grosso op.3/1 with its opening movement in B-flat and its conclusion in G minor...)
Indeed, given that they are from so very early in Schubert's career- they are the first two sonatas in the Dover edition, I think- one could as well ask why he never returned to them (well, the answer is obvious, but one could still ask.)