You don't want a diesel hybrid which has to run DPF and Adblue. Basically Euro6 was very harsh on low to mid mileage vehicles.

The bigger problem is that the Japanese successfully sold vehicles like the Prius and Insight with hybrid drivetrains, and even got some neat patents out of it.
And VAG sold the.... 1,2l TDI Jetta, mostly because their entire lineup is centered around making a very good 1,6l TDI engine that can somewhat comfortably entertain t he idea of going on the autobahn.
If VAG had made their Prius competitor and where a bit more serious, there would have been a diesel euro 5 hybrid to benchmark how good you can make fuel economy.