They're already struggling to meet emissions in a cost-effective way with a 2.0 4-cyl. Also, IIRC, the engine bay isn't quite wide enough for a timing belt equipped 5-cylinder.
And, VW's discontinuing the 2.5 5-cylinder TDI for automotive applications - the Crafter was the last hold-out for the 5-cylinder, and the 2.0 16v BiTDI makes more power and more torque than the single-turbo 2.5 10v.
So, now, if you want more displacement than a 2.0, you need to step up to a V6, which means longitudinal, which means Audi (or a Touareg).