Yeah, I never really got why either. It seems so much more awkward to to make the technique comform to the Satipatthana-sutta and ideas about momentariness than to the anapanasati-sutta, but it probably has to do with being connected to the Burmese lay-meditation movement in the beginning of the 20th century and to the particular interpretations by Burmese scholars. Just a vague non-professional guess
Also, Leigh Brasington teaches the body scan technique (a'la Goenka) as an alternative to his usual ways of getting in to his version of Jhana (i.e by focusing on the nostrils and piti
). Do a search for 'body scan' on his talks on http://www.dharmaseed.org
and at least one talk should come up.