Why did the reformation occur? What really brought that great division in Christendom?
The reasons were primarily spiritual and theological, I believe. The Reformation's understanding of salvation, religious authority, and the church was a major break with the theology officially espoused by the Roman Catholic Church. The Protestant Reformation was then, first and foremost, a theological revolution.
The present-day evangelical attitude toward the place and importance of theology in the life of the church is a major reason why the teaching of the Reformation has been blocked out in the church. Theology today is either considered a necessary evil or something that is, practically speaking, irrelevant to the concerns of ministry and church growth. Lip service is sometimes paid to the reformation, but hardly more than that.