Supply is much, much smaller and those bikes seem to have a reputation. As much as I think they are cool, to me they are old school fun that doesn't justify a premium price, in fact quite the opposite. Probably a common view given that many of the high-priced XR's were on the market for a long time.
To give you an idea of the market scale: creeper started a thread "for 2005 640 Adventure owners, all 200 or so"...in the USA, that is. There's only 2 in NZ.