On: Acme, a brief history of one of the protocols which has changed the Internet

[cortesoft]

You can just read their explanation: https://letsencrypt.org/2015/11/09/why-90-days

Tl;dr is to limit damage from leaked certs and to encourage automation.

[ChrisArchitect → cortesoft]

Related recently:

Decreasing Certificate Lifetimes to 45 Days

https://news.ycombinator.com/item?id=46117126