Use of 6 digit instructions & readbacks is an ICAO recommendation adopted by Eurocontrol. Believe me, the UK Phraseology Working Group looked at ways to avoid it, but to put it plain and simple, if a particular country has just one 8.33 khz frequency, it's required to comply; the fact that the USA does not notify any 8.33 khz is why you do not have to comply in the US. Why 8.33 and not 12.5 khz (which would have been the 'natural' next stage)? Who knows, but the French uptake of these channels might give you a clue.
There was also some evidence that with some 8.33 khz radios, the need to switch from 25 to 8.33 physically meant that an inadvertant selection could take place thus pilot changing from one to the other without switching his radio causing loss of communications and eventually scrambling of Air Defence fighters.