What was the author's intention in writing this particular book?

David McCullough's intention was to highlight the acts of heroism and bravery which ordinary Americans demonstrated in bringing the American frontier to the west. In focusing on the trials and tribulations experienced during the settlement process, he raises awareness about the sheer difficulty of undertaking such a task and thus forces readers to acknowledge their own realities and their gratitude for the work their predecessors did. In a way, the fact that the story of the Reverend Cutler and his Ohio mission was relatively unknown prior to the publication of this book adds to McCullough's immense task in uncovering a particular piece of history and making readers aware of its lasting significance, however invisible that significance may be to the common eye.

Why might author David McCullough have chosen to rely so heavily on primary sources in his book?