Both the big picture and the details are equally important to focus on. I think accident investigation boards are better at identifying the details, which are more tangible and more easily describable. Such as some logic trap in this or that combination of submodes, or a seldom-encountered interaction of A and B systems... things that can be pointed to and shown that happened in a particular moment on the flight. And these can be (and should be) fixed so as not to contribute in a future accident.
But why the pilot acted or failed to act in a particular way? That's fuzzier and harder to identify. It has to do with trends, habits, and mental states. It's harder to identify as happening at X moment in time and setting off the chain of events Y and Z. And that could mean it's left out of the accident report, even though it may be just as much (or even more of a) contributor to the accident than the easily pointed out details from the last paragraph. It's identified not for X moment in time by the accident investigator, but over many flights (or decades of flights) by conscious pilots, instructors, etc., especially when they can see these trends and changes spanning over long periods of time, or over different sectors of the industry, as they observe habits and inclinations.
The thing is, no matter how carefully we cinch up every identified hole in the logic traps and system interactions (i.e., the details) a new unforeseen one is always liable to show itself in the future and be best (or only) curable by a flight crew with a grip on the big picture (i.e., the airplane is going THIS WAY, and I'm simply gonna make it go THAT WAY, with the readiness and ease of an instructor saying "I have the controls" to a pre-solo student)