Porsche has always been considered the most extreme brand, and one of the only brands with a versatile sports car.
While Mercedes, BMW, Volkswagen had many good, and reliable vehicles none of them were as reverted as Porsche in Europe.
Now... Jumping back in time, the Euro was worth $1.65 at its peak 7-9 years ago. This meant that you could take a vacation in sunny California, buy a Porsche so cheap that the shipping, and import tax didn't matter and you could even make money after you have put close to 20 thousand miles on the car.
This was possible due to the old timer / classic car rule in countries such as Holland, Germany, and France. Although all of their import tax rules are slightly different you could import a car 30+ years of age anywhere from 4-7% VAT (Value Added Tax).
This movement became very popular, and caused a huge vacuum of classic cars being exported from the US. Porsche along side the 65-68 Ford Mustang was one of the leading cars.
When the owners of the longhoods, SC, Carreras made money selling a car they didn't much care for they felt fine about it. Jumping back now, most of the Porsches have been siphoned out back to their motherland.
From this point forward the market will stagnate, and there is a chance that the Porsches will come back.
Why the new trend yet again?
The Euro crashed when Greece borrowed Billions of dollars to rescue their country.
The Euro is currently at 1.09 which is the lowest it has been in over 20 years.
There are Porsches which will increase in value regardless of the world market due to the fact that there are still very limited production numbers for certain specific cars.
The first 911 (64-65) will be leading the stock F body pack with the RS trying to break through the stratosphere. The 3.0 Litre Carreras (pre-78SC) will be leading the G body wagon although the first production Turbo will continue to blow the seams off of doors. The RS / 993 Turbo and Turbo S will also continue to rise along with the 959.
Hope I could shed a little light on the question.