Well... Porsche did the 914 and 924 in four-cylinder form.

The Cayenne is, and always has been, a cash cow for Porsche. All that matters to them is that they sell scads of them, so that they can keep developing new 911s.

That's not to say that it's a garbage car. Far from it... the luxury level of even the most basic Cayenne blows away the Mercedes-Audi-BMW contingent.... and the off-road performance of the Cayenne is pretty good. But still, if we're talking diesels, there are much better buys in the class simply because Porsche doesn't see fit to offer a competitive option.