Because the lunar disk is not smooth, and all it takes is a brief flash of sunlight pouring over the disk of the moon and you could be blinded. During totality, there is no reason why you could not view it with the naked eye for a few moments...but I wouldn't push it. The setting sun is hidden by atmospheric extinction and this is a safer situation.
All answers are provided by Dr. Sten Odenwald (Raytheon STX)
NASA IMAGE/POETRY Education and Public Outreach program.