Why You Cannot Use a CNAME Record at the Root Domain

The The DNS  administration interface doesn’t allow it to be added to the root of the domain because the CNAME (canonical name) is not only a redirection of the record A, but also the full domain name. If a CNAME record could be created for the domain, it would also move the the NS and  and SOA records elsewhere, which would be equivalent to making the domain unusable.