This happens even outside Defense in all specialized industry. I was once asked if I could reverse engineer a part for a 30 year newspaper stacking machine in a printing press. Very interesting stuff and a consequence of being niche.
- Limited production run. These aren't Ford Mustangs or Toyota Priuses with hundreds of thousands and millions being produced. There were 21 built over 13 years. This also leads to the consequence that each one should be considered a bespoke creation and not a "copy" of the others (even ignoring the 20+ years of individual maintenance work they've each had).
- Time. It's been 21 years since the last one was produced in 2000. Whatever facility produced this has long since lost that capability.
- Aging workforce. Whoever designed it is likely retired, and could even be dead at this point. Certainly the senior engineers who may have been 40+ when the project started in the 70s/80s. Even if they weren't retiring and dying, the people on the project have been doing other things for 20+ years.