For some reason people fixate on the mitochondrion in the context of the origin of eukaryotes, to the near total exclusion of the nucleus that actually defines

For some reason people fixate on the mitochondrion in the context of the origin of eukaryotes, to the near total exclusion of the nucleus that actually defines the eukaryotes. I don't understand why this is. I can only cynically guess that it's a form of streetlight fallacy: we have a pretty good idea where mitochondria came from, so let's focus on that (and conveniently forget about the nucleus, golgi body, endoplasmic reticulum, ...).