Memory Alpha gives this explanation: http://memory-alpha.org/wiki/Hodgkin%27s_Law_of_Parallel_Planetary_DevelopmentHodgkin's Law of Parallel Planetary Development
The theory was that similar planets with similar environments and similar populations tended to gravitate toward similar biological developments over time. Although initially applicable only to biology it was later expanded to include a tendency to move toward similar sociological developments as well with sentient beings.