The contributors to the Guardian symposium on this question all missed what I think is the main reason: people who buy (and may also read) books in the US (at least) love fact-filled disquisitions about the long sweep of history. Remember Guns, Germs and Steel? On a more econ-ish note, This Time Is Different? The length and factual detail of Capital in the Twenty-First Century, combined with its multi-century and multi-country scope, makes it that much more attractive.
If Angus Maddison had ever written a tome for the general reader, it would probably have been a mega-success.
In other words, while the style and politics of Piketty may have played a part, these aspects alone predict thousands of best-sellers. Other filters are more important.
Of course, luck always plays a role too. Avoid overfitting, especially when n is very small.