I think one of the reasons for the increased timescales is the use of computers for systems design, testing in the development phase and control in the air vehicle itself.
If you take the Buccaneer as an example, when I started at Brough in 1970, Mechanical Systems, as a department in its own right was relatively new. Until only a few years before it had been just another part of the Design Office and during the main Buccaneer design and development programme, the Draughtsmen, Mechanical and Electrical Systems engineers were all co-located. The logic of a Buccaneer fuel system is relatively simple and can be defined in systems diagrams and simple text which any engineer could understand. It was straightforward for the required information to pass from the fuel systems specialist to the electrical systems specialist who would design the logic into a wiring diragram that the fuel man could understand and check. Although many of the individual components had to be specified and bought in from suppliers the requirements specification was reasonably easy.
Step forward 30 years or so and the technical departments have grown in size and become more insular. Take another Brough example of introducing nosewheel steering on Hawk. Now the mechanical systems engineer has to specify the logic of the system in great detail, subject to all the internal design authorty checks required. This is passed to the electrical systems engineer who writes the specification for the steering control unit. This again goes through internal checks before being passed to the vendor who will design and manufacture it. Here the logic now has to be transformed into a software specification. The software designer who has probably no practical experience of aircraft systems operation or support, codes the software. Here you have a major disconnect, because the mechanical systems engineer can't read or understand the software code. The result is the need for a really exhaustive check on the software in its own right and then again when its operating on a development rig and finally in aircraft ground and flight test. Even this process cannot uncover every possible combination of logic or likely fault in the system so there are still lots of surprises waiting to come out when the system goes into service.
Modern aircraft and systems designs are much more capable and sophisticated than those of the Bucaneer era. Unfortuately even in the strictly technical aspects of the design, this results in much longer timescales. Throw in the requirements for greater documentation, health and safety, product liability, project management and customer contractual requirements, it's not surprising that timescales have increased.