Because Ruby understood his obligations as a member of the mob. He wasn't an unstable nut like Oswald.
Ruby wasn't mob. He was very much an unstable nut. Read his own testimony.
Ruby drove around with a dog in his car. He called the dog his wife.
People that knew Ruby say that he was unstable and that this is what caused him to make the uncontrolled decision to shoot when he saw Oswald's smirk in the police station.
Ruby was never a member of the mob. He was a night club owner.