A first work sells for the same reason that any other work sells, it catches the eye of someone that is in a position to make the sale happen. For a work from a known writer, the name of the author is enough to catch the eye. For a work from a relatively less known writer, it has to be something else.Somebody starts reading the manuscript and after a few pages thinks, I would buy this if I were just browsing in a bookstore. Meaning, to that one person, it would be one of the top three or four books in a typical store.
This is a very high bar, and there is a lot of luck involved. Readers have different tastes in literature. And whether your manuscript gets read by a person that really finds the work speaks to the heart or someone that finds it to be unutterable garbage is a matter of chance.
Short answer? Books stand out because they stand out to the people that happen to read them. If no one ever is going to think that your book is nearly the best thing ever, then it simply will not sell. Doesn't mean there's anything wrong with the book, just that it doesn't belong to the paraeto set of current manuscripts, for each and every reader in the publishing business, there is another book that person would rather buy.