To be blunt, there is no real answer to this that would be adequate or concise whilst covering all aspects. Also, I'd separate the arms of Protestantism into Lutheranism and Calvinism, as they were very different.

Mack P. Holt's book on the French Wars of Religion does a fantastic job of explaining why certain areas of France converted and some did not. His basic argument is that there aren't any overarching factors and that you have to look at every case separately. Generally though, he talks about Calvinism and about it being a 'bottom up' conversion.

Lutheranism is a whole different field. It's also easier to answer your question solely referring to it. Princes and towns that wanted more control over their lands 'converted' to Lutheranism. It was an out from what they saw as Italian oppression in Germany. Lutheranism gave them a reason to have more financial independence from Rome. Michael Mullet's biography of Luther gives a good account of why it took hold in Germany.

As a very general hypothesis (which could be wrong, I would like to be corrected if so) weaker kings/ rulers were the ones to convert. The king of France had a very independent church (1516, Concordat of Bologna) as did the king of Castille. They saw no reason to break from Rome. Germans, Sweden, Denmark, England etc all had relatively weak rulers (king of England had ~3 million subjects vs the king of France's ~20 million) and so they wanted a way to achieve the same level of autonomy as their counter parts. Hence, they became Protestant, stopped paying tithes and ripped up church land for themselves.