#2. There's enough time in between to build him up

While we are beginning the most important period in the WWE calendar year, one of the reasons why WWE is in no rush to debut Sullivan is that they feel there's enough time in between to debut and build him up.
Think about it. There are two PPVs for some odd reason between Royal Rumble and WrestleMania 35, and that alone states that there's more than enough time to debut him, book him as a monster and build up a solid storyline for the grandest stage of them all!
#1. Shuffling creative plans

WWE creative can often be chaotic, and due to a variety of circumstances, plans have to constantly be changed. As mentioned, it's about to be the most important period of the year, and creative needs to have a solid set of plans for each superstar.
While the original plan may have been for Sullivan to face John Cena, it's likely that creative is making a few back-up plans just in case of any issues. As a result, they'll hold him off until they deem him ready to debut.
Becky Lynch has been challenged HERE.