Fact 5: Dean wanted always to be dead, he was the one to jump always he heard the word "SACRIFICE" he had a lot of guilt with him and regrets we can noticed on episode "Unity" he couldn't continue with all those feelings...

Fact 6: Sam wanted to have a real life you know? have a family, kids and a dog even, since the beginning of the show we knew that...

Fact 7: If Dean would die in another produced way you know another God, archangel, angel, demon etc...Sam was forced to revenge that dead and continue that story, again that wasn't the point of the #SupernaturalFinale so this dead had to happen with a simple accident without help