The main reason for the cancellation of the MRA4 is money. Despite all the money and effort expended on this late and costly project, BWoS would not provide a support contract that MOD could afford. These discussions have been going on for years and BWoS still wanted blood and more profit. They thought the project was too big for the Government to pull the plug on. Wrong! It is the future costs that broke the bank.
PS. There were no issues or delays with the mission system software. This was delivered some years ago to specification by Boeing. It just didn't have a working airframe to fly in.