I think the problem is that the number of people who are interested in lots of different subjects isn't very big. And if you include the need for those overlapping circles to include "conrunning," then you reduce the number down nearly to the empty set.

Heck, I'm fond of lots of different things, but nobody has infinite resources, so I have tended to specialize in events like Worldcon and Westercon. But I try to at least attend other related-field events like FurCon and WonderCon.