Flight system avionics (for aircraft or missiles) go through some grueling QA/QC, V&V testing so that's why builders don't upgrade very often and try to get maximum efficiency out of every RAM bit and CPU cycle.
That's why
AP-101 series are still used though the design is about 40+ years old. Most missile avionic gear is "unique" but is typically based on a MIL-STD 1553 bus architecture. Equipment details are usually classified.