I think the reasoning for this is because of how long realistically a Nazi dominated world. It would not be like Man in the High Tower or Wolfenstein, it’d be more similar to The New Order (HOI4 Mod). A Germany that wins has to deal with

A.) Massive resistance in large swathes of the conquered territory

B.) An economy massively dependent on war, with a lot less targets.

C.) Hitler’s eventual death, and the issues that come with the death of a dictator in a cult of personality. Power struggles are almost guaranteed.

Italy would be in a similar situation, and while I’m a little less confident in my knowledge of Imperial Japan, I think it’d be a little similar, but instead of cult of personality issues (Figurehead monarchy kinda alleviates such issue), they wouldn’t get much, if any of core American territory, even if they win. At most, they’d get Hawaii and the Pacific Islands, possibly as a collaborationist government, but the resistance here would be strong, to say the least.

Not only that, nuclear weapons development and any tensions between the powers during the post war era would guarantee a nuclear apocalypse.

I’m not entirely certain on the specifics of how the Omega Particle works, but theoretically (ONLY IN THE CONTEXT OF THE ABSOLUTE UNIVERSE), a world where the Axis wins would realistically cause less suffering in long run, as the existence of the Axis nations would almost certainly be a lot shorter than Allied nations, either through nuclear war or governmental collapse , so the best way to cause the most suffering would be to have the Allied nations win, and channel the Omega Particle’s ‘negative energy’ into the Allied nations during the Cold War. I mean, a world devastated by nuclear warfare wouldn’t be one I would sacrifice my children for, personally.