Fear dictates their decision-making more than God's Word and His promises. And it all goes back to sin. Fear is the result of sin, just as it was in the Garden of Eden. After Adam and Eve had sinned, when God came looking for the beloved crown of His creation, they were hiding. The Fear you feel because of sin puts you in familiar company.
Fear is a result of sin.
You do not need to fear anything else. He could say this to Abraham, even though he was a sinful man, because Abraham had the right kind of fear. We hear of that in the account of Him being told to offer his only Son, Isaac, as a sacrifice. As difficult as that was, Abraham was ready to go through with it. Do not do anything to him.