There were a number of reasons for the rapid spread of the Reformation.
One of the most important of these was technological. This was the invention of the printing press, which had happened about a century ago. By the time of the Reformation, printing had become very common in Europe. This allowed works explaining Protestant ideas to spread rapidly over Europe.
Of course, there had to be a climate that was receptive to the ideas in order for them to spread effectively. This means that another reason for the spread of the Reformation was that the people of Europe were ready for its ideas. This can be traced to the growing corruption in the Catholic Church and the feeling that it was not doing a good enough job of providing for the spiritual needs of its members.
In this way, technology allowed the ideas to spread and social attitudes meant that the ideas came to be widely accepted. This allowed the Reformation to spread rapidly.