Because it was a stupid premise that was going to get people’s butts in seats for one reason and one reason only, and that was to see the Alien fight the Predator, so they could literally do whatever they wanted with the rest of the movie, they didn’t care, so what did they do?
Put Sanaa Lathan in as the lead, gave her a shield made of an alien’s crest and a spear made of an alien’s tail and said “go fight alongside the Predator to save the world from an Alien Queen”, and that’s what she did. They didn’t have to do that, they could have had a white guy, they could have had a white woman, a black man, even, but no, they went with a black woman and made her incredible. And when she fought her way to the end of that movie, they made sure that the most dangerous hunters in the galaxy recognized her and respected her, and they marked her because they decided “You are a paragon of your species and we honor you as one of our own” so in this universe when the Predators think of the Best that Humanity has to offer, they imagine the face of a black woman of science with long curly hair.
…did you ever watch the movie Predator…and then have a nightmare where you are sent into the jungle as part of a commando team and then hunted by a brutal alien creature with superhuman strength and the ability to disappear into its surroundings…but instead of an alien it’s a loaf of bread… a Breadator?
After Rocky IV’s release, a joke was making rounds that since Rocky had run out of opponents, he would have to fight an alien in the fifth installment. Screenwriters Jim and John Thomas took the joke seriously and wrote a screenplay based on the joke which became the movie Predator.