Before I get into the answer...I want it to be known that I've had a few and my answer might not the best in terms of grammar. Okay, my resources are "The Four Horseman of the Apocalypse: Religion, War, Famine, and Death in Reformation Europe" by Cunningham and Grell. My second source is "The Reformation: A History" by MacCulloch.

Number one, Mr. Doctor, you have to understand the time of the Reformations in which we can situate ourselves in relationship to the popularity of Calvinism initially. The Reformations era is a time in which we have to think about how people viewed religion and theology. In the social history of the Reformations theology is so intwined with how people thought and reacted to the world (MacCulloch 110). The reformations were an apocalyptic age in which its people thought that they were legitimately in the Last Days with everything from war becoming endemic across Europe to crises in disease and famine being read from an eschatological and apocalyptic point of view (Cunningham 1-3). Thus, whatever type of Christianity that you were in became paramount as YOUR salvation was at stake. If you believed in the wrong Christianity, you were fucked...damned. Luther (as you can't have a conversation of the Reformations without talking about him) and other reformers defined the deceiving antichrist as The Pope himself (Cunningham 4). So, Luther himself by the protestant propaganda machine was portrayed as an eschatological prophet (Cunningham 24,25). Calvin himself was convinced that they were living in the last days (Cunningham 27).

Two, predestination wasn't new in the reformations era. Lutheranism has a type of a predestination in its own theology as Luther was an Augustinian monk who was deeply influenced by Augustine. To Luther, God set aside his wrath as a judge for an elect group of beings. This was a gift, essentially (McCulloch 115, 120) and if we go back to the discussion of Apocalypticism, you needed to be sure you had the right Christianity to be in. If the Pope was the antichrist, deceiving the world and if good works--a mechanism towards salvation under the catholic system--wasn't the way to said salvation and a product of the popish antichrist then you are fucked. Calvin just added double-predestination as a remix (MacCulloch 243). Thus, remembering the whole talk about the reformations being apocalyptic, the appeal of Calvinism was that people thought it was a good way toward salvation. It wasn't nice.

As far as why it caught on...there are reasons for this but I'll just talk about why I think Calvinism is so popular. One of the features of Calvinism was its militantly separatist children...The Puritans who came to what now is Massachusetts and, in lesser numbers, to the Caribbean (MacCulloch 535). In both Cunningham and MacCulloch (though I've forgotten where) they talk about the militancy of these early Calvinists and how they found themselves in a lot of places (like, for instance, the French Huguenots). Another aspect of Calvinism that was attractive was the concept of Providence. This is the notion that god governs all events (this part came from "Calvin: An Introduction to His Thought by THL Parker on page 19). So, with the aforementioned militancy and the very real nature that Calvinists abroad (abroad meaning outside of Geneva) were fighting for their existence (see French wars of religion for example) and that this Providence would give them hope that with their election, the future looks bright (again, this sourcing comes from the amalgam of the different books that I am coming from...I just forgot the places where I saw them)! There are some moments of hope in Calvinist theology.

Anyway /r/DoctorEmperor, I hope that helped! Again, if I have grammar mistakes, Jack and I would like to apologize ;)