At a guess, I'd say the reason is because Let's Encrypt (the service) didn't exist when they got their certificate.

From Wikipedia:

> Let's Encrypt is a certificate authority that launched on April 12, 2016

From Let's Encrypt's certificate

> Period of Validity > Begins On: 2015/2/4

I agree they should eat their own dogfood, but in the meantime, I'm eating their dogfood and it tastes pretty good.