1) The want to represent the book that will make them the most money, not necessarily the best book.
2) If your bok doesn't fit their focus, it doesn't matter how good it is, they won't publish it.
3) Relationships matter. Knowing someone in a publishing house doesn't hurt.
4) Publishes like when you have your own platform to sell your book since their budget for marketing is not a quarter what it use to be.
5) The big advance rarely happens anymore for first time authors.