Don't judge the Cayenne based on what people think in Germany.

This car is a hit in places(markets) where no other Porsche would have had a chance of registering sales.

Also the reason for diesel in the case of Porsche is to lower consumption and emissions, so they wouldn't install a huge engine for this purpose.

VW/Audi developed these big diesels for "diesel image" i.e. in order to sell lots of TDIs on their normal models. Porsche doesn't need a halo model diesel.