Reformers before Luther had said comparable things and had been contained. Wycliffe was condemned and Hus was burned. Explaining why 1517 did not end the same way requires more than theology.
Printing is the usual first answer and it is a good one. Luther's German pamphlets were short, vivid and cheap, and they sold in numbers no earlier controversialist could have reached. He is among the first authors in European history whose output can be described as a publishing phenomenon, and the woodcut allowed the argument to reach people who could not read.
Political interest is the second. German princes had long-standing grievances about money leaving for Rome and about the appointment of bishops, and a reformer who denied papal jurisdiction was useful. Frederick of Saxony protected Luther without ever clearly endorsing him. Later, monastic property became available to rulers who adopted the new order, which is not a cynical observation but a documented one.
Existing grievance is the third. Anticlerical complaint, demands for reform, and devotional movements seeking a more personal religion were widespread before Luther and were not created by him.
Historians weight these differently, and the weighting is genuinely contested. What is not contested is that the Reformation produced many churches rather than one, and that the magisterial reformers persecuted the Anabaptists on the radical wing as vigorously as any Catholic authority did.