Henry VIII converted England to Protestantism for one critical reason: he needed to secure a divorce from his first wife in order to remarry. All of the king's efforts in this regard were trained on the ultimate goal of obtaining a marriage that produced a son and viable heir.
In the earliest phase of the Protestant Reformation, Henry VIII was actually one of the Catholic Church's greatest and most vocal supporters. In response to what he saw as the theological heresies of Martin Luther, Henry produced a highly eloquent defense of the church that ultimately earned him the title, "Defender of the Faith," from Pope Leo X. However, events turned when Henry's aging first wife, Catherine of Aragon, failed to produce a son. Now in love with Anne Boleyn, and wishing to divorce Catherine, Henry approached Pope Julius II for an annulment of his first marriage.
The pope, however, was in an extraordinarily difficult position. He found himself and Vatican city under the occupation of Holy Roman Emperor Charles V, Catherine of Aragon's nephew. Predicting that his allowing Henry to shamefully divorce Charles' aunt would likely cause the further deterioration of his predicament, Pope Julius dragged the affair out interminably, ultimately forcing Henry to take matters into his own hands. Through aggressive political maneuvering, Henry finally convinced the majority of the English clergy to name him sole protector and Supreme head of the Church of England, thus giving him full spiritual jurisdiction over the realm. Consequently, Henry was able to orchestrate the divorce he so desired.