Currently we can not tell WHY people unsubscribe. There is no report to generate about the why. MaillChimp had that feature.
Feature request is to add a drop down with reasons unbsubscribers can pick from as soon as they hit the unsubscribe link from the email’s footer. There should be no reason why they need to hit a custom link to go to a different page to explain why they unsubscribed.
Thanks!