DNS servers can't answer more than one question per query because NXDOMAIN is signalled in the message header and so when QDCOUNT is greater than one the response becomes ambiguous.
> 1. Some hosts might want you to use a CNAME record to send them your web traffic, and CNAME records are undesirable for apex domain names because they can't coexist with other records you might need (like MX).
Nitpick: With the exception of RRSIGs, a CNAME record must be the only record at a given owner name. Since the apex of a zone has a SOA record, a CNAME cannot exist there.
They didn't want to wait for multiple queries to finish. If you're interested in a detailed rationale you can take a look through the mailing list archives (though it'll be frustrating reading).