Why is it more dangerous to look at the sun during total eclipse than in watching the sun set?

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.