One of the most important reasons is the lack of an oxygen-rich atmosphere needed to support an Earth-like organism. Most living things here on Earth require oxygen in order to survive, a gas that the moon doesn't have a lot of. That's not to say that the moon doesn't have any atmosphere at all; it actually has one but human beings and animals here on Earth can't use those gases to survive. It's possible that a creature that breathes other types of gases like potassium from another planet can live on the moon, but we unfortunately haven't found evidence for the presence of such creatures. In addition, the moon's atmosphere is really, really thin compared to that of the Earth's. So it almost doesn't matter. I hope that answers your question. Good luck with your studies!