I forget the details, but I think the argument intentionally focuses on ancestor simulations for epistemic reasons, to preserve a similarity between the simulating and simulated universes. If you don't assume that the basement-level universe is quite similar to our own, it's hard to reason about its computational resources. It's also hard to tell in what proportion a totally different civilization would simulate human civilizations, hence the focus on ancestor simulations. I'm not sure if this is a conservative assumption (giving some sort of lower bound) or just done for tractability.

ETA: See FAQs #4 and #11 here.