It seems to me that after doing the medieval thing five times Square finally decided to try to see if RPGs, and Final Fantasy in particular, worked outside of the medieval-type setting. Turns out it can work anywhere if you do it right. They decided to come back to it with IX after they had established that it worked elsewhere, and my guess is, if they continue the series, we will see a mainly-medieval setting again.
And for the record, I love the sci-fi/medieval-fantasy fusion, but I'm a little concerned that the setting of XIII is essentially XII with perhaps a more technology-based/evolved world. Granted, I haven't seen much of either game so far.