Traditional Survivor Series 5 on 5 match gains unpredictability
If Reigns was part of Team Raw, it's pretty obvious how the match would be booked.
Almost every booking decision nowadays revolves around how to get him over as much as possible before WrestleMania 34 that there is no way in hell that the team he is part of, Team Raw, is taking a loss at Survivor Series.
And having invested so much time into the under siege angle and Kurt Angle's job being on the line and all that, it makes little sense if everyone knows which team will win heading into Survivor Series.
Even in the age of the internet where there are next to no surprises and secrets anymore, that's just taking it too far.
All things considered, it's better for the pay-per-view as a whole if Reigns isn't involved in that match at all.