One thing that causes dist. shaft wear, and especially on the older long shaft distributors, is not changing the oil often enough or allowing an engine to severely overheat.
There’s a small hole underneath the dist.base and the bushing is lubed by splash oil essentially. If the engine has lax oil changes or overheating the hole will plug up due to coked motor oil. After that the bushing starts going south.,
In some cases the advance mechanism may seize or throw a spring or the vacuum advance (if so equipped) will fail.