Image Source: Marvel Digital Comics
Moon Knight was actually an Avenger for a cup of coffee before he went completely crazy with the multiple personalities. After this, no one wanted to be associated with him. Even the Punisher thought he was crazy. He couldn’t decide if he was going to be the type of hero that kills, cripples, or incapacitates. He was also taking orders from his god Khonshu that we don’t know exists or not. As time went on, he got his act together and became a legit hero. Even his crazy was toned down a bit. Once he reinvented himself, Steve Rogers came and asked him to be part of his Secret Avengers.
Moon Knight was such an odd member because he is so unpredictable. Even when he got his act together, there was no guarantee that he was going to stay sane. There was proof in multiple series after Secret Avengers where he went back to having multiple personalities. In one story he even had Wolverine, Captain America, and Spider-Man in his brain talking to him.
Cap says, “Once an Avengers, always an Avenger” but it’s doubtful Moon Knight will be an Avenger again any time soon. This works out better for Moon Knight fans. He’s free to be the crazy person we’ve accustomed to. Plus, he’s not much of a team guy.