Why doesn't Automated Certificate Management support wildcard domains?
Currently, ACM doesn't support wildcard domains as mentioned in the known limitations section of our Dev Center article. Supporting wildcard domains is something we'd like to do, but unfortunately this limitation comes upstream from Let's Encrypt. They don't issue certificates with wildcard domains in the domain list. They have a discussion thread in their community forum about this if you're interested. We won't be able to support wildcard domains until Let's Encrypt does.