Television, case in point, The Death of Superman came about because the team on the comics had planned for Lois and Clark to get married in the books but then found out that was going to happen in the Lois and Clark show on television in the future.
Wanting to have the comic book marriage coincide with the television marriage, the team decided to postpone the comic marriage but was left with what direction to take Superman that year in the books.
This lead to the decision to kill Superman off and we got…The Death of Superman.