It was definitely a concious decision, but not for the reasons you think. It was done because it saves time/resources. If they actually made Kassandra believable they'd probably have to rewrite entire dialogues and quests from the Alexios playthrough. Because as a man/woman pairing they'd experience it differently.

Look at Mafia 3 - that's an example of historical accuracy and a minority character done correctly. Lincoln was black, an awesome character, and was being called the n-word throughout the entire game. But it made the journey feel so raw and truthful.

I'm happy the develope's didn't just hand-wave it away by saying "black people experience racism in real life, therefore why subject Lincoln Clay to it"