Why?
The Protestant Reformation was the result of a complex mix of religious, intellectual, political, social, and economic factors. But the real heart of the Reformation can be attributed to none of these factors. It was not a revolt, intended to overthrow the establishment or institution of the church. Nor was it motivated merely out of dissatisfaction with the corruption within the church. It was not a mere moral cleanup project. Nor was it the religious side of the Renaissance. Nor was it a human rights movement to correct abuses. Though each of these causes may find something to sing about in the Reformation, none of them were its aim or ambition.
The truest aim of the Reformation was the glory of God, through a right and proper worship and devotion to Him in His church. To be sure, this involved cleansing her of her manifold corruptions. But such was not the driving ambition behind the Reformation. Seeking the purity of the church is good and pleasing to the Lord. Rightly, other so-called reforms have sought it. But the Reformation is more fundamentally about the gospel itself breaking into the heart of man and drawing him in the newness of life.
Why the Reformation? Because God is worthy of right worship through Christ crucified and risen, and man is desperately needful of grace.
See also: After Darkness, Light