the requirements change faster than the snails pace of procurement
Some years ago, after yet another uninformed rant by the Public Accounts Committee, it was suggested that the "customers" (Service users) be invited to comment on the best and worst procurement projects they'd experienced. After all, Post Project Reviews are written by the project manager, not the customer. Few bothered (itself a feature of procurement), but analysis of the successful projects revealed some embarrassing similarities and recommendations, most of which involved stripping out 3 or 4 layers of upper "management" involvement from both sides. By far the most successful RN project was notable for the RN's Aircraft Support Executive withdrawing support for it before the contracts were even let, leaving MoD(PE) to do the job properly in peace.