#4 Not much excitement in other competitors winning
All competitors in this match are credible opponents for the current champions in the industry but for some, it may be too early to hold the Universal or WWE Title. Considering how hot Rollins and Kingston are right now, them losing their championships would only hurt their run. In order to keep their run with the Titles intact, an opponent needed to be chosen who is well established and keeps the crowd coming back for more.
Finn Balor has the Intercontinental Championship to his name and having him win the briefcase would be too draggy and uneventful. Orton is a legend in his own right and has nothing to prove anymore. Ricochet, Andrade and Ali are all fresh in the WWE and them losing to either Rollins or Kingston would only cause a roadblock in their character development.
Corbin is a wonderful heel but he has been pushed to lose against a lot of Superstars. McIntyre vs Rollins would have been a match to watch but McIntyre doesn't really need the briefcase to consolidate his position in the roster.
Given the current scenario of the Champions, Lesnar challenging Rollins makes the most sense.