(Click the chart to make it bigger) Each column is a 1000km slice. There are obvious reasons for the steps in it. The second column being bigger than the first is skip. Sub-1000km QSOs are rarer because of skip. The first step at 5000 km is the Eastern seaboard of the United States coming in. To the left of that, there's a gentle fall off as we progress over Europe (bear in mind I've popped all the bands into one pot here).
The second step is as we cross the relatively barren and difficult lands of India and China to Japan, thence to Australia and New Zealand, who lurk off on the right hand end. 20,000 km is about as far as you can do until SETI makes some discoveries.
So, clearly it gets more difficult with increasing distance (no surprises there). But it's also influenced by the difficulty of working DX in certain countries because of the lack of amateur activity there. There are, for example, only an estimated 800 hams in PR China - whereas there are 1.3 million in Japan.
But then, 800 is more than the total number of us involved in WSPR (about 500).