Why isn't the HL7 standard using XML?
HL7 version 3 is using XML. Earlier versions of HL7 are a health care specific derivative of a pipe delimited format (IIRC its called ER7, not sure though).
Theres nothing particularly wrong with the pipe delimited format. In many ways the newer XML variations are harder to use. It may be old, but for the most part it works and it is well understood.
Looks like v.2 was created in 1987 according to wikipedia: http://en.wikipedia.org/wiki/Health_Level_7
That's probably your answer.