I'm not surprised it's selling well, but it's a bit weird that it took so long to take off, when you consider how long the big GSs have been around, and they've been such sorted bikes, right from the start.
IIRC it was about 2007 that GS sales suddenly tipped and went from slow-burn to ballistic. Go for a ride anywhere in Western Europe now, and GSs are everywhere!
The 1200 is a very modern bike, but the 1100, which came out in 1994 has the same core qualities of character, practicality, versatility, comfort, ease-of-use, fun, and beaky quirkiness.
Maybe there's just more average disposable income these days to pay BMs premium prices.