The reason might be that the race would only work with one lap with the current race durations (short is around 8 minutes, medium is around 12 mintues). It might also be that the timing with qualifying doesn’t align when joining a Nordscheilfe lobby coming from a previous race, leaving you with only a couple of minutes.
There hasn’t been a clear communication why it isn’t in the game, but there’s definitely more people that want to see it.
A quick search finds multiple suggestions for the Nordschleife to be added, with the first one having over 80 votes already. Be sure to vote on it and let T10 know why you wish to see it!