With the recent "Superstar Shakeup" there were a couple "surprises" regarding moves that didn't happen. One was AJ Styles not going to RAW and the other was Roman Reigns not moving to SmackDown.
The plan to move Reigns to the blue brand was being discussed up until late, but the folks over at the Wrestling Observer Newsletter noted that moving Reigns to SmackDown would have also required them to move Braun Strowman and Brock Lesnar as well. Why you ask?
Well, because the WWE has summer plans for all three of those guys involving Strowman going up against Reigns, leading to Strowman v. Lesnar - all while keeping Reigns v. Lesnar rumbling while keeping those two apart as long as possible. As the plan is still, at this point, to have Reigns v. Lesnar at WrestleMania 34 next year!