Apparently, Bill Burr was slated for an appearance back in the Breaking Bad universe, but the timing didn't work out for personal reasons. If he had appeared when Kuby would have fit into the story, then he wouldn't have been able to say goodbye to somebody in his life who was dying. Who can blame him for making the choice he did, as nice as it would have been to see Kuby in the flesh again on Better Call Saul?