Because the content inside is 5 stars easily. It's a wonderful book, highly recommended. But what I am very disappointed to find is that neither issue 0 nor Annual 2 are present, as described here (AMAZON, FIX THIS). It certainly doesn't bother me as much as many, but I'm pretty sure a product being sold with a false description is cause enough for some to seek refunds, and while this is a lovely book that isn't suffering in content, the fact that it's advertised as having a lot more content is wrong.
Doesn't matter much to the story, Annual 2 as loose fitting in this story, I hated the artwork and the story didn't grab me. Also, I think issue 0 was effectively retconned by Snyder's work in these main four issues. I was curious as to where it fit into this story in his mind, but this book seems to answer this question with "not at all".
Anyway, the 4 issues that remain (one of which is double sized) are 4 of the best issues in comics in a long time. This is such a good reimagining of Batman's story, clever use of ambiguity as always with Snyder's Batman, and they work on their own as a single story arc while setting up for the next part of Zero Year pretty well. I love this book to bits, and it very easily stands amongst the best Batman stories, alongside Court of Owls and Death of the Family. My favourite is Court of Owls, but I'd have to see that this story is the most successful of his work, because thus far, Zero Year has achieved the impossible; retold Batman's origin in a new and interesting way while staying faithful AND being deferent in every way. If I had predicted the best possible outcome of Snyder redoing the origin story, it wouldn't have come close to predicting this. Snyder gave us fans pages of a story none of us even knew we wanted until we saw them.