Wasn't it simply because the people who wrote WFRP2 were American non-GW employees? GW just licensed the rights and walked away IIRC
That's probably part of it, yeah. But the original WH40K was written in the 80's which was not a nice decade to live in Britain (Unless you were rich, and games designers aren't); basically summed up as "Thatcher and the Cold War, everything fucking sucks", and by the 90's, a lot of that was gone. The designers just weren't thinking like that any more.
Warhammer, 40K, Fighting Fantasy, SLA Industries; all very much products of their time.
In the original RT book there's a two page splash of muties... all of them based on the design team.
I actually like that picture. It's silly but not
stupid, and it's explicitly a shout-out to the developers.