Our current CDN doesn’t make it trivial to provision wildcard domains and certs, so it wasn’t something we were able to provide. It’s possible with our new provider this may become an option, but at this time we haven’t made any decisions about which provider we will be using so I can’t say for certain.
With LetsEncrypt, would a wildcard cert even really be necessary? You could just generate a cert for a new subdomain whenever a new user publishes an HTML game for the first time. They do have rate limits but I imagine the public one could be workable, and if not, one can always contact them to work out potential rate limit increases (there's a form on their rate limits page for it).