It was appropriate in Prometheus' case because, while it was the same world as the Alien films, it was a new story not featuring the capital-A Alien.
Once they decided to shift gears in the sequel and redirect it into a more direct prequel to the original film, the "Alien" returned to the title and we got Alien: Covenant.
With Predator sure, you can call it "Skulls" or whatever, but Predator doesn't really offer a world in the way that Alien does. There are stories in the Alien universe that you can tell without the Alien. In the Predator universe though? You really aren't doing much of anything without featuring a Predator, and at that rate, you might as well use the name in the title.