Why did the Protestant religion appeal to kings?

Probably because Protestantism did not tie them to Rome and make them subservient to the Pope. The Pope had the power to excommunicate anyone, including kings. I know of at least one Catholic king who humbled himself to the Pope rather than be excommunicated. Of course, a Protestant king was still answerable to God, but he need not have worried about another man sentencing him to eternal damnation.