#2. Dean Ambrose was wasted on SmackDown last time
WWE made a huge mistake back in 2016 when they decided to award Dean Ambrose the WWE Championship at Money in the Bank, allowed him to retain at Battleground and then made him the main star of SmackDown Live.
Ambrose is not the kind of star who is able to lead a brand like AJ Styles could and being on a brand with The Phenomenal pulled up all of his flaws and left his WWE Championship reign looking like one of the weakest in history. Ambrose doesn't have fond memories of SmackDown Live, so it's probably the best option to keep him on Raw.