Too open a frontier and groups like the Mongols tended to ruin your life. The Chinas and Russias of the world tended to do pretty badly. Too rugged an interior and you could never achieve enough cultural unification to put everyone on the same side. No one wanted to be Persia or Ireland, constantly struggling with internal discord. The goldilocks geographies were those with solid, crunchy outsides and gooey centers: England, Japan, the Ottoman Empire, Sweden.
