Because there were 10 survivors left who could say where it actually happened. Because Pakistani civilians witnessed Blackhawk helicopters hovering above the gunfight (one witness told the news that he knew they had to be americans, because Pakistan doesn’t operate Blackhawks). Because a big chunk of blown-up Blackhawk was left at the scene. Because a house with a couple of bodies and big blood stains was left behind…
…in other words, there was too much evidence of a major seizure to be able to even consider keeping it secret.