The reason for that is because they want the entire roster backstage at Raw so that will allow them to include some of the Smackdown talent on Raw. This also will help squash some of the leaks from getting out since everyone, instead of a small group of Smackdown stars, will be at Raw. Most, if not all, of the wrestlers, do not know if they will be switching brands. Usually, they are not told until it is announced on TV. Only the top names like Roman Reigns would be told beforehand.
The early word we got was that they were looking at moving Alexa Bliss and Mickie James to Smackdown Live. Also, based on the advertising for the Extreme Rules pay-per-view, it looks like AJ Styles, Baron Corbin, and Shinsuke Nakamura will be staying on Smackdown Live.
Click here to see the advertisement for the Extreme Rules pay-per-view.