Warning- Extremely gay overanalysis incoming.
even if they wanted to (they obviously don't), portray Super Earth as anything other than a perfectly equal mix of all races, ethnicities, creeds, and sexualities. That's the reason why.
I think presuming too much about creative decisions is rolling the dice but I'm going to do it anyways, at risk of being contradicted by some obscure lore stuffed on a computer terminal in the background. I would actually think enforced diversity would suit the image that Super Earth projects of a united front of humanity, with an air of legitimacy, so that there's no actual excuse to not be part of the Federation. The sole intolerable "human" separatists to be forcibly locked away where nobody could see them were transhumanist cyborgs.
Do I think that's actually the source of the decision? No, that's post hoc reasoning, but Helldivers isn't exactly subtle as long as you look under the papery veneer of what's presented as diagetic media and party lines.
As far as the Ministry of Truth is concerned, the discord of racial disharmony does not exist and if you imply weakness in the bonds of humanity then the Truth will be enforced upon you, and as far as the Ministry of Defense is concerned, the average readiness rating of a Helldiver is only about 20% of the ideal anyways so they're going to stuff the hellpods with niggers and shoot them out and if they kill fifty bots or bugs they've paid their worth, since tactically speaking a Helldiver is basically a forward observer on bath salts and the actual hammer is the Super Destroyer's guns and its Eagle raids. Or rather than bath salts, Stims, which are definitely not addictive substances to supercharge eminently expendable soldiers with just enough immortality to get the job done.
Even though Super Earth is supposed to be the 'bad guys' (As oppose to the wild bugs, robotic killing machines, and equally genocidal aliens), the concept of portraying any faction, or any group, or any ideology that does not conform to this principle is flatly impossible. It literally cannot be done. Doesn't matter if you're portraying "Nazis x1000" they still have to have an equal ratio of all of the above. Anything to the contrary is simply impossible for Sony, Arrowhead, or any of these leftist companies to fathom.
The funny thing is that even though the sorts you expect describe Super Earth as "fascist," like I've said, Helldivers isn't a subtle setting, and there's really no reason not to assume that Super Earth isn't a real Democracy, with the "Managed" part being the guiding hands of the various ministries. Some people can't fathom that a Democratic system can be made into a totalitarian one, but all that's really going on in Helldivers is the various branches of government and the military industrial complex compiling a Super Patriot act while everybody smiles and gives their consent.
Obviously though there's no real choice to do otherwise, you're either with Super Earth or you're being eaten by bugs, at best enslaved into a collective machine consciousness by automatons or enslaved by them, or being made into an ash ghoul by squids. The debate of whether all these things are just the sins of the father coming home to roost are irrelevant for anybody alive in Helldivers 2 anyways. You don't get to pick sides in race wars no matter how they started, not if you're actually involved in them.
And if there's no choice but to fight for Super Earth, then why not be a Helldiver, who has a short life of glory where the entire Federation cheers you as a celebrity and you have your own ship? Just don't be disillusioned by the presence of other Helldivers sardine packed in the ship ready to take your place the moment you fall.