Complex sentence: Johnson felt Martin had failed to record the more interesting aspects of life at the time and thought this was due to Martin being unaware of just how different the social structure of the Western Isles was in comparison to the modern world.
Split into simpler sentences: Johnson felt Martin had failed to record the more interesting aspects of life at the time. He though this was due to Martin being unaware of just how different the social structure of the Western Isles was in comparison to the modern world.