Remember when the 993 Twin Turbo's came out? Used ones were holding their value insanely well. They barely dropped in price! It took a while before they came below $90k or so.
I think production numbers come into play as well. Porsche didn't seem to make that many Turbos in years past (only two years of production on the 993 Turbo, '94 3.6's are rare, '91-'92 cars weren't built in huge numbers, etc). Now, 996 Turbo's are plentiful and the new 997 Turbo is out without much waiting between models (996 to 997). So, supply exceeds demand and pricing reflects that verses years past.
The 996 Turbo wasn't the old insane animal that Turbos used to be. I've not driven one, but several people I've talked to that have had decent seat time in them (and are familiar with older Turbos) state that while very quick, they could be used daily with little trouble or stress. More GT in feel than raw sports car. Maybe that affects what the used enthusiast market thinks of the car as well. The GT3 was always the more raw car and maybe enthusiats that have $65k to blow on a car may just pass up the "cushy" Turbo and go for something more raw...
In my area near my work, I used to routinely see a woman in her 60's driving a silver (did they all come in silver??) 996 Turbo around in town at 25 mph. I'm sure it was an automatic and was bought for status, not performance.
Jay
84 3.2, 90 964