BUT BUT BUT,
The legions don't exist anymore (despite the Primarchs leading them on realspace raids.....), and it makes more sense for the unfettered, uncountable hordes of chaos warbands out there to all work EXACTLY THE SAME.....
Just like it makes perfect sense for those 10000 year old veterans to be worse than their 300 year old counterparts...senility and all that.
And finally this all ties in with the fact that despite being excommunicated for innovation the dark mechanicus can't figure out how to change up the weapons on the standard raider/rhino....whereas their ritula bound brothers in the regular mechanicus crank out different variants every couple of thousand years....
/stupid logic
realistically theres not much we can do except hope that it gets fixed in another 7 years....maybe, if any of us could be bothered with this **** anymore.