Because someone else played the part of Jerry’s father first.

Because they recast the role later with better comedic actors after the show had become popular enough for them to attract and pay those better comedic actors. In some instances, they even went back and reshot scenes that took place in previous seasons and inserted them into the reruns and DVD releases, deleting the old actors entirely. (Most specifically with George’s father)

Or because the original actor was hired for a one-episode contract, and when it came time to have a parent appear again, the original was unavailable.