When the idea of roster moves was brought up a couple of months ago, the original idea for major moves was Roman Reigns and AJ Styles switching brands. Neither ended up switching and WWE likely decided to switch Bray Wyatt over instead of AJ Styles at the last minute.
As for Roman Reigns, because of WWE’s current and future storyline plans, moving him to Raw would have also likely meant that WWE would need to bring Braun Strowman and Brock Lesnar along with him, according to Dave Meltzer.
As has been rumored for a while, the current plan is to have Strowman feud with Reigns, then have Strowman take on Brock Lesnar (likely in June or July). The rumored WrestleMania 34 main event is Roman Reigns vs. Brock Lesnar, but that’s a year away, so we’ll see if WWE actually tries to keep them apart for a full year or if they have them exchange the Universal title a couple of times along the way.