A bit late to answer here, but...
This is a throwback to the early days of MIDI. There were standard controller assignments for a lot of things (list here
http://www.indiana.edu/~emusic/cntrlnumb.html)
#6 - Data was just that... lots of synths had a 'data entry' slider or wheel and to keep some semblance of a standard, they all mapped to 6 just for consistency.