Michael Richards & Jason Alexander were also not in “The Pen.” This caused Jason to approach the producers and he told them that if they produced another episode without George, they’d be producing all future episodes without him too. All he asks is for a scene, or a small b-story to remind everyone involved that this was an ensemble. From that point forward, all 4 characters appeared in every episode, even if they didn’t get a story (one which comes to mind for me is The Busboy, where Jerry appeared in the episode, even though he wasn’t involved in either the A or B story, except for the first scene when they’re eating at the restaurant, and the very last scene when the busboy and Elaine’s boyfriend meet on the staircase)
The problem with Kramer in the first 2 seasons is that he was a character who never leaves the apartment building, which caused him to have to sit out of any episode that didn’t take place there. This is why they did the multiple episode arc of him moving to LA after being liberated from Jerry’s apartment when his key was taken back.