The reason behind Flipkart deleting negative reviews is very obvious, they dont want people reading about a customer’s bad service experience. Here is their review moderation process - https://blog.flipkart.com/deletin. According to the link, they approve positive and negative product reviews. However, when it comes to a review that highlights negative or positive aspects of their service, they reject it. Their blog says, "They reject positive service reviews on product pages too". This is what the link says verbatim, "If it’s a positive service feedback, mail back the customer thanking for his testimonial. If it’s negative, forward it to Customer Support(and follow up)." Now there are two ways of looking at this,
Publishing Negative Reviews - If someone buys a product from Flipkart and the product is faulty or not what has been promised then the customer sends a negative review. He/she can send a positive review depending on the experience. Now Flipkart published both, positive and negative reviews focused on a product. I think their rationale is, if a customer is coming on a product page, he/she expects to read reviews about the product. If the positive and negative reviews focus mostly on service then it defeats the purpose of having reviews on a product page.