Why?
Less than 25% of the original frame was used, each frame was different. Each frame had to be backward engineered to a common spec to take the new wingbox. By the time the reverse engineering was completed a good part of the pressure shell had been replaced.
Remember, the original order was for 21 frame's, 3 test & 18 production, as the requirements decreased the cost's went up. The cost of making jigs for 21 frames was significantly cheaper than making 9 individual frames and has been said before BAE could have marketed the MRA4 to other NATO countries......not going thru with this option was madness in its own right
The madness was trying to use scrap to build a new frame.