Rogue Trader was mostly 60s-80s scifi stuff the creators liked stapled onto Warhammer.
2000 AD comics, Dune, Alien/Aliens...etc...
Then the Heresy was just Paradise Lost.

So you have "Fans pulling together ideas and settings they liked and mashing them together so they could play tabletop games with a similar flavour" vs corporation trying to emulate bigger IP holders to break into the mainstream regardless of it's own history, inspiration and previous fanbase.