Urriak Uruk
Gaming is fun, and fun is for everyone
Fewer Factions than Theros has gods. Most people will only have to think about their characters Faction, and their specific rivalries. The point is that they are a story engine, thst can give form to Adventures in a very wide open space.
Like I said before, Theros doesn't need to cover nearly as much ground as Planescape does. If anything, the writers of Theros were struggling how to fill more pages with material. Planescape will have the opposite problem; what material can they put in the book, and what needs to be cut.
Now, I'll concede that if the factions are tied explicitly to specific planes, then it makes more sense to give them material. The Dustmen for example could serve as a dual-purpose section on not just their faction, but on how to survive and run adventures in the Negative Energy Plane. But if they are used (as they largely have) as just factions in Sigil, I think they need to be consolidated or given sparse detail.