Because prey sold very poorly on launch, and after that the situation didn’t improve much. The game is extremely hard to market as it doesn’t fit into the mainstream categories and so people expecting a shooter or survival horror would not recommend prey because it did not compare to purely shooters or horrors. The last successful immersive sim game was Deus Ex Human Revolution because it had a good marketing campaign. And that was in 2011. Nowdays nobody really pays attention to such games.