Genetics, not knowing HOW to train back correctly and the lack of flexibility in the shoulders
It seems that the guys who have a lot of back detail train back with a squeeze motion and perform pull ups correctly. Like they get a real good contraction of the scapula, and they also train the lower back well. Look at Flex, Shawn, Ronnie, Dorian and others from that era with good back detail. In today's era look at Phil and Kai train back, they even trained together at one time. A pull up or lay pull down mimics the Abadan pose, so doing a lot of those helps in the mind muscle connection as well.
Now, take a look at the back training of those who have terrible BDB poses and lack detail. Their training is telling a la Dillett, Nasser, and Jay Cutler.