I wonder if it's the difference in BMW's perceived reliability and actual reliability. Years ago German engineering was excellent but then they seemed to embrace complexity in all their products and reliability suffered but their reputation lives on.
https://www.google.co.uk/search?q=r1...+drive+failure
https://www.google.co.uk/search?q=r1200gs+head+shake
https://www.google.co.uk/search?q=r1200gs+abs+failure
I wondered if the servicing was expensive (so people sold before they were due) but it doesn't seem that bad although valve checks were every 6000 miles. You see a lot of Street Triples sold before 12k for that reason (12k service was about £650 on the 675).