"The Joint Strike Fighter acquisition strategy is designed to meet affordability goals by reducing program risk before proceeding into the engineering and manufacturing development phase. (never happened EP) To that end, the acquisition strategy is designed to ensure a better match between the maturity of key technologies and the aircraft's requirements. Matching the requirements and the maturity of technology when a program enters engineering and manufacturing development (development phase) is a critical determinant of a program's success. Once the development phase begins, a large, fixed investment in the form of human capital, facilities, and materials is sunk into the program and any significant changes will have a large, rippling effect on schedule and cost. Beginning the development phase when critical technologies are at a low level of maturity serves to significantly increase program risk and the likelihood of schedule delays, which in turn result in increased program costs."
Bold emphasis mine.