At least, one possible reason: the fields were established recently enough that someone with a very general set of knowledge from all sorts of different fields (i.e., the kind of knowledge Ne types excel at) can contribute a significant amount of new and constructive material. As time goes on, fields get more specific and require a greater amount of specialization to come up with something new in that field. Ne-dominant types don't like to specialize too much (there's so much else they could be doing), so they lose interest and move to something else where they can contribute more (including concepts they just learned in the other field).