As I understand the regulations, RX Supercars have to respect certain measurements of the base car, not general limits like in WRC. VW Motorsport used the 2014 Polo anyway, because it was basically ready to race. They did some development back in 2015, just in case. The only car available at that time was the 2014 Polo WRC.

Also there will be a new Polo roadcar in summer 2017, which obviously they couldn't use just yet. I wouldn't be surprised if there will be a totally new Polo Supercar in 2018.