I believe (but cannot prove) that the 20% is mostly due to China. Because it's such an incredibly populous nation, it can dominate international statistics like these.

Unfortunately my home nation (though very small) is also on something like 20% WinXP (thus limited to IE8). It's ridiculous, but as long as clients are willing to pay us to support IE8, then that's what we'll do, much as we hate it. In my country it hopefully won't be long before it becomes prohobitively expensive to support IE8, so there's that... :)