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Fan fiction is much easier since it already has an established universe, characters, and audience. Hence you don't really need to do a lot of world building or character development, since everyone's already familiar with it all, and you don't have to build up a reader base, since it's already there.
...You also don't need good writing or a coherent plot. Nor do you need consistent characters. Just slap a famous character somewhere and create some sort of ass-pull romance and you'll be famous in no time.