Every now and again, there is a DC story so massive that it shifts the very fabric of reality. We tend to call those stories "Crises" for a couple of reasons.
Okay, can we pause for a second? I feel like I'm missing a very crucial piece of information here. Despite my lack of experience reading comics, as I've made clear in weeks prior, I consider myself pretty knowledgeable about Superman.
Passing superhero mantles between people can make things...complicated. The pressure of upholding a legacy, the potential jealousy, maybe even feelings of being replaced by a newcomer.