That would be because you set up the HTTP-01 challenge as your domain verification method.
https://letsencrypt.org/docs/challenge-types/
Since there is no equivalent HTTPS way of doing the same thing?
You can use DNS-01 or TLS-ALPN-01 if you don't want to (or can't) open up port 80.
Since there is no equivalent HTTPS way of doing the same thing?
You can use DNS-01 or TLS-ALPN-01 if you don't want to (or can't) open up port 80.