As far as I see it, the aircraft were not ready in time. There is now a first aircraft in full Sundair colors undergoing mods in Abu Dhabi:
http://www.ostsee-zeitung.de/var/sto...rticleWide.jpg
The delay has been partly caused by the last-minute switch from A319s to A320s. The two A320s they were able to source are ex LATAM aircraft.
First flight is now scheduled for July 20. In the meantime, they are using third-party aircraft. There will only be two aircraft for this summer season, based at TXL and KSF. The FRA base has been scrapped and FRA-passengers are now booked on Condor. The third aircraft will join Sundair for the summer 2018 season.