LetsEncrypt is what finally solved the problem with the previous systems were perfect was the enemy of the good. There used to be a lot of focus on trying to verify that the person getting the cert was who they said they are, but LetsEncrypt came and sad "that part isn't important, protecting the data is what really matters" and basically solved the real problem wholesale. Very few people inspect the certificates on every website they visit, so those protections were mostly useless and created a roadblock to widespread adoption of HTTPS, scoping the problem down to just making sure nobody can inspect or modify the data in transit made automation not only possible but easy.