Rear tires wear

I have a 1999 Porsche Carrera cabriolet and my question is that I cannot put more then 10,000 KM on a set of rear tires. Is there a problem with my car or my driving (I do not track or race with the car). I have tried Bridgestone, Yokohama (265/35/18R) without any success I still have to change the tires around 8,000 or 9,000 KM which is not a lot. I try to purchase tires with the hardest gums but no positive results. Does somebody have the same problem, if there is a problem or is this a normal situation? There is no problem with the front tires, normal wear and tear. I agree that most of the weight is on the rear axle and it is a rear wheel drive but still 9,000 KM is not a lot!!! What brand if tire should I purchase to have a better mileage with the car?

Thanks & have a great day

Yves