If a private key leaks, a shorter certificate lifespan limits the fallout
Faster upgrades to new cryptographic standards. If some of the crypto methods used get broken, short lived certificates means they get replaced with newer methods faster
Short lived certificates force sysadmins to automate the renewal process. This prevents expired certificates due to forgetting the manual renewal.
Certificate lifetime and domain ownership mismatch. You could buy a 2 year certificate for somedomain.com and then sell the domain or just let it expire and have it picked up by someone else. You then have a valid certificate for a domain you no longer ow and you could MitM traffic for the new owner’s website.
Several reasons