As the title says, how come so many of the early successful civilizations sprouted from the northern hemisphere, and why to this day are the majority of successful (economically, socially, etc...) nations in the northern hemisphere?
Everything else aside, the land area of the Northern Hemisphere is significantly greater than that of the Southern Hemisphere, about double. Just based on that you'd expect there to be roughly twice as many civilisations.
However, Antarctica makes up a huge portion of the land in the Southern Hemisphere, and is uninhabitable. Looking at other land masses, we need to see how much is likewise uninhabitable, or at least not capable of supporting large populations. The Australian Outback is a good example of such an area, and covers the majority of the continent (Australia is nearly the size of the United States but the continent has a population of only around 25 million, about the same as the city of Shanghai).
The answer to your question is simply that the Northern Hemisphere simply has way way more habitable land, and therefore a much greater potential to support large populations.
The Southern Hemisphere simply lacks the land.