I believe it stems from JW theology, which only addresses true/false, right/wrong, light/dark, us/them, happy/sad, chaste/immoral, moderation/excess, etc. Subtlety and nuance have no place in JW culture. However, life is full of these finer elements, as is great art.
As art is a reflection of life, so JW art is a reflection of JW life. JW art is as artificial as the life it reflects.
I have personally witnessed wonderful music created and then shot down because it fell outside the boundaries set by those leaders steeped in JW theology and culture.