How many people can play those kingdom songs competently nowadays? Talent dies off, and unless it's replaced, you have to go for the easier solution.
This gets to the heart of the 'problem' the WTS had at the time. Of course, the problem was all of their own making, but more of that later. Many congregations at the time were 'stuck' with one ill-equipped pianist available to play the songs at the KH. Worse still, he/she wasn't of the humble type i.e. the elders didn't want to upset their feelings by telling them that they were simply awful at playing the songs. Yet there seemed no-one else to take their place.
C.O.s from around the country reported on the standard of singing the songs and that something should be done. The answer, as always, was to blame the publishers and so "simplification" i.e. eradication, took place and music by Muzak took over. It's been that way ever since.
As far as real singing is concerned, I find it hard to recall a time in my 50 years a dub on two continents where I could say I heard "real" singing at the KH. Loud singing at times, like when the song and music were meorable but I wouldn't say "real" singing. Over the past several years I have heard real, heartfelt, joyous singing at christian events and it certainly hasn't been at a KH!!!
More than that, why don't witnesses sing? A friend of mine, a minister and author, visited a KH last year for a meeting. He wanted to see what they were about. His conclusion? "They can't sing", he said, "but that's not surprising, they haven't got God's Spirit, so how could they sing; they've got nothing to sing out for!" Makes ya think, eh?
Anyway, why was all this a problem of the Borg's own making? Put simply, they have never encouraged people, especially the young ones, to learn music - real music that is. After all, we well know the WTS' opinion on extra learning. . So they were left with their "new nation™" - a nation of zombies, who looked alike, dressed alike, and ideally, for the borgmeisters, were alike. After all, the time was short and the young should be out there "witnessing", not 'wasting' time learning how to play the piano. If, on the rare occasion, a witness youth excelled at their musical studies, it would never be acknowledged, let alone praised or encouraged. So the WTS faced the inevitable problem - very few witnesses could play a musical instrument to a competent degree.