Maybe they just don't know. Maybe there are some proofs of soft bodies creatures having lived on Mars and NASA does not want to scare people, these organisms could be dangerous, toxic or just be some kind of Alien (like in the movies lol with sigourney weaver, alien covenant , predator vs alien, etc)).
Or maybe NASA is ruled by aliens (I means kind ones not monsters that just want to kill and eat us) and they do not want us to know. Who knows ...