One reason is to increase capacity. Another was a more environmentally friendly option hence the reason they went for the A321neoThere are sufficient aircraft sitting in desert locations and given that Jet2 have made use of an AirTanker A330, Why do they not secure one of these redundant craft? I would ask the same question to TUI.
The air tanker is still currently active. A reason why they don’t take aircraft sat in the dessert is because most are old planes which are not economically viable to operate anymore or they still belong to airlines in question but they are being stored there as the airlines don’t require the aircraft for the time being due to travel demand being low.
Also there probably isn’t any aircraft that suits them sat in the dessert.