I'm gonna guess it's all of those but Gruul instead of Rakdos.
One, the Rakdos and Dimir have both been malign shits in both prior visits but not the actual villain causing problems either time, so I think the Rakdos are going to play against type the same way the Dimir seem to by not being cartoonishly evil this time around and actually helping out.
Two, Rakdos is an idiot but he's still one of the strongest bastards on the plane and his followers are a cult rather than a tribe bound by his strength. I feel like while neither of them could possibly take Bolas, Bolas would have a much easier time knocking down Borby and proclaiming "what I say goes now," and the Gruul will follow while the Rakdos would not fall in line so well.
Three, while they are unpleasant giggling sweatgoblins whose continued existence is a threat to every INDIVIDUAL person in Ravnica, the Rakdos have, in their own special way, kept the city as a whole safe by keeping Rakdos entertained so he doesn't skull-fuck the plane. The Gruul are actually pretty OK with the plane getting skull-fucked at this point.