It just came out at an appropriate time; a year after Occupy Wall Street, etc. Inequality and inter-generational mobility is clearly on people's minds these days.
Similar story happened with Nassim Taleb's Black Swan book. It's not a particularly good book -- it's rambling, hand wavy, and only ever presents anecdotal evidence as support for its claims. But the book came out at an appropriate time (right before the financial crisis) so it was embraced.