I have no idea why the TV shows and movies do that, Tia. Maybe it's because of how dramatic and emotionally powerful the death of Glenn Ford's Jonathan was in the original movie?

When did post-Crisis Jonathan die? The 1990s? Man, I thought they'd let him live. I guess that must've happened after I stopped buying comics. I know that Jonathan was alive during the Death of Superman storyline in the early 1990s.