I would guess because the whole region is surrounded by mountain ranges which must have been impassable "jungle". E.g. the Ore Mountains in the north were only settled after 1000 AD and mostly because of various "silver rushes". I guess the Bohemian forest to the west was similar harsh, no large rivers like the Rhine to transport troops on, and the whole area probably a scary, dark and "barbarian" wilderness like Germania north-east of the Rhine. Just speculating though :)