My guess is it has a lot to do with how easy or not it is to start your own thing. In my mind, developers in the US are more likely to have options to strike out on their own, so they need more $ to work for the man.
I always assumed it was the opposite -- mainly because of healthcare. I think the various 'entrepreneur indexes' generally rank the US high, but I'm not sure i buy it -- just b/c of the healthcare thing. i would think, ok, right off the top, you get rid of anyone 30+ years old, anyone with a family, any women, anyone with any kind of health problems, anyone who is not a super-high-rish-taker, etc. So that leaves the…
Now, with the ACA, you can get meh coverage without the extensive shopping and hoop jumping it used to require.