I don’t know either (yo no se tampoco)
But I can imagine an answer.
Generally, products have ads that show how good they are.
When customers are satisfied with the product, what they feel had been already expressed in the product advertisement. They don’t have anything new to tell to humanity. Unless they think is awesome and it is much better than the ad. Then they express a good review.
When customers are not satisfied, there is a truth that no one had said before, or they want to add their voice to those who contradict the advertisement.