The problem is that Let's Encrypt doesn't support wildcard certs, so having a single cert for the origin and allowing connections on "www." is not possible. This is a problem because a request on "https://www." will be rejected completely rather than redirected to the origin (or vice versa). In other words, I have to choose one, and the other one won't work at all and can't be redirected (for https, but I'm auto-redirecting from http to https as well, so for everything). Obviously, the marketing gains from not having a "www" outweigh any other considerations at this point, so no "www".
As I understand it, anyway. I could be wrong. I hope I'm wrong, and have just misunderstood how this all hangs together.