Reasons
1. Tessa Thompson is amazing and looks iconic
2. It's great for new fans, most of whom will be confused by a blonde, white Valkyrie in the comics
3. Marvel's independent films don't really have an iconic black woman since storm is with fox.
Has Valkyrie even been seen since the Hickman run of the avengers? Since the universe hiccuped you could just have the new Valkyrie appear and no-one in the comic book would need to bat an eye