It's partly explained by all the non-US contributors here. That's my theory.
Of course, billionaires are unpopular even in the US. Yet, as sparsely attended at that (earnest!) pro-billionaire protest in San Francisco was, I find it totally unimaginable that that could happen anywhere outside the US.
Most software developers are not founders, but they like to hang out here for the news and community anyway. It used to be a lot more libertarian back when I joined (even more so when I only occasionally lurked) but things have shifted rather dramatically over time.