Consider the scenario where I own a domain example.com for a year. Just a day before its expiry or just a day before I sell the domain to someone else, I obtain a certificate for it from letsencrypt via ACME protocol.
A week or month from now, the new owner of the domain sets up a HTTPs website. With the old certificate I have, I can now launch an MITM attack on the new owner for about 2-3 months!