Hi there, ISRG co-founder and current board member here. In brief, shorter lifetimes force people to automate (which, e.g., avoids outages from manual processes) and mitigates the broken state of revocation in the Web PKI. That latter point especially is what I understand to be driving the Web PKI toward ever-shorter lifetimes.

I actually remember the discussion we had in ~2014 about what the default certificate lifetime should be. My opening bid was two weeks -- roughly the lifetime of an OCSP response. The choice to issue certificates with 90 day lifetimes was still quite aggressive in 2015, but it was a compromise with an even more aggressive position.