Some test jets are so 'heavily' wired that it would be difficult to make them operational due to non-compliance with current approved operational configuration.
For example, the USAF recently retired a CV-22 to its museum after it was deemed to be too different to the operational birds' config to be brought up to that standard...probably had a few hours on it too!
The other issue with test jets is, although they typically have lower total airframe hours on them (the two RAAF ARDU F/A-18s are by far our lowest time jets, despite one of them being the first built), they spend more time at the edge of the performance envelope and thus have a very different fatigue life index.