Ever noticed how someone looks attractive from behind but then they turn round and are not so much? It seems to me that when our brain fills in the gaps it assumes that the subject is aesthetically pleasing. (Well, either our brain defaults to expecting something to look nice, or it fills in the blanks with an average and we find the average to be attractive.)
So coming back to simplistic design and why it evokes a positive emotional response from us: Perhaps it's because with a simple design our brain has more 'blanks' to fill in.
If anyone knows of a name for this phenomenon and further information exploring it I would love to learn about it. Same goes for if what I'm saying is nonsense and there is an alternate theory disproving mine!