The reason is very high demand which surpases the production facilities of Volkswagen's factory.
The reason is unknown and a bit strange since Volkswagen GTE plug-in hybrid is not on highest demand in Europe with just around 7.500 orders over 11 months in 2017. Obviously market predictions were even lower than that.
Similer problem happened also with battery electric model e-Golf, which found arround 12.000 buyers over 11 months of 2017. Volkswagen had to double production of plant from 35 to 70 cars per day or arround 2.100 per month to get closer to demand.