The same reason BA are taking delivery of new A32Xneo/350/787-10s, Virgin are taking A350s and TUI have a load of second hand 737-800s painted up ready to go. These aircraft were ordered and contracted months or years ago, just because COVID happened, you can’t just walk away from the deal without hefty penalties. Manufacturers and leasing companies have been accommodating with delivery delays and deferments given the circumstances, but still... its very difficult for an airline to walk away from their commitments at the last minute.
I’m sure every airline globally who had aircraft on order is stuck with some they’d now rather not have, Cityflyer is hardly unique.