Because he gave away the ending. Seriously, why do authors do this? Yes, it was a well-known, rather old horror novel that's been made into a movie (twice). But I had never read it (um, that's why I was reading it!), and if I hadn't caught on early enough, this famous author would have spoiled the entire novel for me.
An analysis, especially a blow-by-blow analysis, of a novel belongs at the END of a book. In an afterward, or an author's note, or an editor's note, or something of that ilk.
Please, dear editors, dear authors, dear whoever-will-be-writing-introductions, don't spoil the book for your readers, even if the book is a hundred years old. Chances are, there's someone out there who has never read it before and never seen the movie. Do you really want to rip away the joy of that twist ending just so you can sound smart about it?
If you want your introduction to actually be read by someone, try introducing the novel, rather than analyzing it, okay?