Even though automated renewals are absolutely superior, as encouraged by LE, they do require some work to adopt. For a lot of businesses, it's easier to have someone manually renew every year or two than to do the migration work of setting up automated LE certs.

Not only setting up, but monitoring.

Wildcard subdomains aren't supported yet (but should come in January [1]).

[1] https://letsencrypt.org/2017/07/06/wildcard-certificates-com...

It starts bearing the question though: Is one CA being over a third of the market a good thing?

Well, a third of the market that is not that important, over all. I don't see banks switching to LE to save a buck or two.

I guess it's mostly inertia.

Many people who don't live in the tech news bubble like we do might not have even heard of let's encrypt, or haven't realized that they could use it. Or the bad old "never touch a running system" mantra at work.

Because some people/businesses are afraid of free things, and others would prefer to not have their certs rotate every 90 days (even if it's done automatically).