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).
And that's because the major DNS servers don't bother with answering more than one question per query. Nothing in the DNS specification limits the number of questions to one. The only down side might be that the response to multiple questions might not fit in the standard UDP DNS packet.
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.
