Board Thread:The Trials of Apollo/@comment-36801889-20181026175724/@comment-1636709-20181027032528

T.V Tropes has a few thoughts on the subject.

Basically the idea is that Commodus is too interested in himself to ally with either Nero or Caligula. The two know that they and the other is power hungry enough that backstabbing is possible, so Commodus is a buffer between the two to keep them from doing something self destructive.