There were two primary factors that led to the Reformation occurring in Germany. These were the invention of the printing press by Gutenberg in 1440 and the political organization of the German States as members of the Holy Roman Empire when Martin Luther nailed his 95 theses to the church door in 1517.
Prior to the invention of the printing press in 1440, access to books by all but the richest Europeans was extremely limited. Most books at the time were also printed in Latin or Greek, languages that the learned clergy and nobles could read and write, but not the commoners. Therefore, even though the Bible was the most frequently copied book in the world, most Christians had never read it. With the ability to now mass produce books, the German vernacular Bible would begin to spread across the Holy Roman Empire at about the same time as Luther's nailing of the 95 theses. The ability for commoners to read the Bible for themselves also supported Luther's assertion in the Reformation that salvation was achieved through faith alone and not works or penance as translated by a priest.
The political climate was also very advantageous for the Reformation. The Holy Roman Empire, mostly what we now call Germany, was a loose collection of German speaking states each with their own prince, duke, or other royal ruling over them. All states were then subservient to the Holy Roman Emperor who derived much of his power from the support of the pope in Rome. The individual rulers of the various states saw support for Luther's Reformation as a method of reducing the power of the Holy Roman Emperor by challenging the pope and the Catholic Church thus increasing their own control over their individual territories.