Why southern France became protestant in 16th c.?

Just to add - I think you might be making an erroneous assumption here, that the rise of Protestantism was associated with urban, mercantile populations. In fact it tended to spring up in societies that were on Europe's periphery and distant from the most urbanized regions. During the high medieval period, the great centres of education, craftsmanship, and urban culture were mostly the Italian city-states like Genoa and Venice, which were of course strongly Catholic.

England and Germany did have some major urban centres and a good deal of mercantile activity, but not in as great a concentration and not with the same sort of urban culture - that's partially why the Renaissance came late in these areas.

Indeed, however this doesn't mean that Northern Europe in the medieval period, or even shortly thereafter, was any sort of cultural powerhouse. The primary cultural centres were the Italian city-states and republics. Hanseatic trade was important but still, Baltic and North Sea trade as a whole was small potatoes compared to the Meditteranean (and the Hanseatic League itself was fairly minor next to groups like the Medici).

Capitalism happens quite a bit later on - between the time of Martin Luther and the time of Adam Smith, alot happens, including things like the discovery of the New World, the colonization of India, etc. The centres of culture and trade shift during this time - access to the Atlantic becomes much more important than access to the Meditteranean, for instance. The importance of cities like Genoa, Florence, and Venice diminishes considerably, while the importance of cities like London and Amsterdam skyrocket.

For the Netherlands I'm sure it was - I doubt they saw Meditteranean trade as very relevant, since they had little opportunity to participate in it. But overall, grain trade on the Baltic was hardly the most important trade in the world, or even in Europe - silk and spices were quite a bit more important, for instance. Not to mention Spanish and Portuguese colonial enterprises. Plus, during the Reformation, the Netherlands had not yet shaken off the Spanish. 

International trade itself, in fact, wasn't even that important during the early Reformation and really wouldn't be until the period of the Enlightenment. Banking was quite important, but that was still dominated by places like Florence and Venice and Genoa until the foundation of the Royal Exchange and the Amsterdamsche Wisselbank in the 1560s, by which time Protestantism had mostly taken root wherever it was going to take root. By the next century, the most important financial centres were Amsterdam, London, and Hamburg - but that's a long time after the main action of the Reformation, it's just before the Enlightenment.

The Protestant countries did indeed eventually become dominant centres of economic and cultural influence, but they became Protestant before that happened.

Yes, but consider how much trade was happening just to support that much piracy ...

Alot of the issues that the early Protestants had with the church simply weren't issues in the Orthodox church. The Catholic church allowed only the Latin gospel, while the Orthodox church invented the Cyrillic alphabet so that the gospels could be translated into the Slavic languages. Neither did the Orthodox church practice indulgences and pardons, which were one of the biggest issues in the Reformation (just look at Martin Luther's 95 theses: the majority of them are about the sale of pardons and indulgences).

True, but the Calvinists are part of the Reformation as well.