Why were communication officers of the the 1950's closer to the 'coal face' than the officers of the mid-1960's onwards ?

Answer, because they were trained to touch type and read and send Morse code just like their ratings were:  they had common practical skills which drew respect from the lower deck.

Here are two files which are made up from the Long 'C' Course Officers Syllabus for their HMS Collingwood module for Electrical and Radar Training.  The first, Long C Course HMS Collingwood 1954.pdf  from 1954, shows a 10 day course and the second,  Long C Course HMS Collingwood 1956.pdf, virtually two years later is for 1956, now reduced to 8 days.

Note the amount of signalling [which was wireless telegraphy orientated] and touch typing they did in 1954 and how many sessions were devoted to Morse instructions in 1956.  If they were subjected to this level whilst away from HMS Mercury doing a short course, what, I wonder, were they subjected to back in the Signal School where their course lasted for many months ? {Regrettably, we don't have that syllabus}.

For those not familiar with Long Course's for officers, which many branches of the Service ran before the advent of the PWO/AWO concept {these more frequent courses were simply referred to as a PWO Course or an AWO Course}, General List Officers of a certain seniority only, were appointed to The Long Course, which was a once a year event in and from each School.  SL and SD officers were not normally selected as candidates during this period.