Because it encourages seller, not just simply delete the model and re-publish it, but to try to contact unsatisfied buyer and offer him help in trying to solve the issue. Think about this - you have deleted old model, fixed the issues and published it as a new model, but your customer was left with the old, potentially useless model with no access to better model. Do you really think it would be fair to also delete negative review, like if you'd redeemed yourself? If on the other hand you would reach the customer, offer him to fix the flaws in exchange in positive review, that would benefit both of you. I think it's only fair that reviews of deleted models are staying on your record.