Just look at the kind of baggage you have to deal with in a canon, in-continuity story:
In a story like DCeased, you don't have to deal with any of that. You can make a story about Jon becoming Superman make as much or as little sense as possible and not destroy what your fellow writers and artists are doing because it's self-contained and stand-alone. You never have to worry about stepping on anyone's toes and offending the collaborative nature of the main universe.