Why was martin luthers teachings so appealing to peasants?

To the lower class of northern Germany in the 16th century, the church was a rather burdensome institution. The Roman Catholic Church at the time sold something called 'indulgences', which were certificates of forgiveness for sins committed in exchange for large sums of money. The peasantry, who were not wealthy but quite religious, found this policy incredibly burdensome as they were faced with the prospect of starvation if they purchased the indulgences, or hell if they didn't.
Then came Martin Luther in 1517. Martin Luther was not pleased with the indulgences system. In his opinion, the indulgences system was exploitative and greedy, and not in the spirit of the Christian faith. He preached that the true way to forgiveness was through faith and repentance, not actions like the purchase of indulgences. This resonated with the peasantry, who were sick of funneling all their money into the church.
Besides his anti-indulgence stance, Martin Luther also encouraged personal interpretation of the bible. This engaged peasants in the faith instead of concentrating all interpretative power in the hands of the clerical elite, which was quite popular. 

The answer is monsoons.