It's a ring around a mountaintop (which makes it an odd shaped circle, doesn't it ;). Which was fine for its original purpose as a testing track, but makes for a very dangerous racetrack that can't be made safe.

The GT cars later got so fast (even eclipsing the F1s) that tracks like Le Mans were forced to put in speedbumps. So the days of the Nordschleife as a racetrack were numbered in any case. F1 may have accelerated it a bit, but I doubt it made little difference.