Stack Overflow has been around for almost nine years, and I don't think anyone can deny that it is an absolute resounding success by any objective standard. There are not very many serious programmers (professional or hobbyist) that do not use Stack Overflow to find answers on a semi-regular basis. This is quite an achievement.
There are 168 other Stack Exchange sites too, but literally all of them are a lot smaller than Stack Overflow. Stack Overflow consistently gets about 8,000 questions a day. Maths is the second-largest site and gets "only" about 500 questions a day, the third-largest is the Russian Stack Overflow site with about 200 questions a day (this doesn't really count), and the fourth largest is Ask Ubuntu with about 150 questions a day.
Quite a difference!
Not only that, in my experience a lot of sites that have nothing to do with IT – such as Politics, Skeptics, Christianity, English Language, etc. – are dominated by people with an IT background – usually programmers. In other words, the already small (comparatively) traffic is fed by Stack Overflow, rather than "organic" growth.
Why? What makes Stack Overflow so special? Is it the topic matter? Something in the Stack Overflow community or the programming community at large? Or is it just chance? Something else?