We often talk about how "privileged" we are to live in the developed world, as if it was just luck that we wound up with a higher quality of life than other cultures/religions/ethnicities.
I can't help feeling that our ancestors must have been doing something right.
The short answer to this is "We don't know".
There is no generally accepted theory on why western civlization developed the way it did. Jared Diamond has one theory in Guns, Germs and Steel but it is far from generally accepted. My personal view is that Diamond presents a heavily Eurocentric explanation that often downplays or entirely ignores non-European factors to make his theory work. Diamonds work has been extensively deconstructed by /u/snickeringshadow in /r/badhistoy here.
My personal views is that there is no one right answer to this. There has been previous and similar threads before, you'll find one such example here although they might mostly serve to highlight the fact that there is no real consensus on the topic.