It requires TCP overhead if the dns packet is bigger than 512 bytes. You could abuse the DNS protocol to carry HTTP-esque metadata but it would be a small victory over standard HTTP. Reusing an existing TCP connection is more efficient.
TCP is not required for larger messages if EDNS(0) is available--which it probably is--and the network allows; which conservatively, it probably does at least till the 1280~1450 byte mark.
