Why could the disciples not recognize Jesus?

Why did the disciples not recognise Jesus when he appeared to them after his death? 

Because after his resurrection, his body was a spiritual body, and their natural eyes could not behold him—when they really saw him, it was with their spiritual eyes.

Insight—A Record of Psychic Experiences—A series of questions and answers dealing with the World of Facts, the World of Ideals and the World of Realities Beyond Death