Hmm...
QDM = inbound course
Same number of syllables so what's the logic in using it in the 21st century?
QDR = bearing from or radial
Same number of syllables...
Methinks it's because the British invented Q codes in 1909 and, just like we still do things because that's how they were done on the (...fill in the blank, DC-4, Convair, TriStar, Classic...) we should still use Q codes because every time a Brit discovers a Yank who doesn't know what QDM is he can then write it in his ERAS!