The maintenance burden

Even within their domains, expert systems became expensive to keep alive.

Every new case required new rules. Rules conflicted with each other. Changing one broke three others. The engineers who built MYCIN described maintaining it as "painting a bridge": by the time you finished one end, the other end needed repainting.

The knowledge didn't transfer either. A system built for diagnosing infections couldn't be adapted for diagnosing engine faults. Each new domain meant starting over entirely.