#1 Seth Rollins is being saved as a WWE Royal Rumble surprise
The New Year's Day episode of WWE SmackDown was stacked and already included the return of Sonya Deville, which could be one of the reasons why Seth Rollins wasn't also added to the show.
With The Royal Rumble right around the corner, the show that has become famous for including returnees. It makes sense for the company to hold back on several stars and wait for the Rumble match.
Seth Rollins would be expected to be a Royal Rumble surprise especially as a former winner of the match and someone who could then pick up a feud if he isn't handed one upon his return.
The former WWE Champion has only been away from WWE for around five weeks and since traditional paternity leave is closer to six, this week was seemingly a little too early for Rollins to make his return. Perhaps The Royal Rumble would be a much better venue for a return for someone of Rollins' caliber which is why WWE made the late change.