Wage growth has been slow in the last few years (tightarses!), and with that, people's budgets for their recreational pursuits (including bikes) has shrunk. Therefore, there is slightly less competition and less people willing to pay more for what is on offer. One thing for sure though, I watched a pristine U.S model XR600RG bid up to almost $12K last week, and an unrestored, but tidy XT500 sell for $6K, so compared to 15 years ago, these bikes are still expensive if they are very tidy.
I guess there are other things at play too, as has been mentioned above, the pool of buyers for "older" pre 80 bikes may be shrinking because 1- Some have got too old to ride and 2- Some are migrating to the Pre-90/Pre-85 classes.
Food for thought.
Cheers, Mark