Not hard and fast...
No, Writer, it isn't difficult to get your novel published, assuming you've actually written one. It does take patience and persistence. I sent out about a hundred queries before a publisher in England picked up my first novel, The Summer Boy. It turned out to be a fruitful relationship. We did four books together over the next five years.
My fifth book, Slick Rock Creek, required only fifty queries to find a home. Maybe I was writing better by then. More likely, I just got lucky, although having a track record helped considerably, I'm sure.
Why change publishers after four books? The short answer is that my editor at Alfie Dog Fiction simply didn't like the story, and required revisions that would have rendered it another tale entirely, one I wasn't interested in telling. Fortunately, the folks at Solstice Publishing had a different take on it, one closer to my own vision.
Of course, I still have to answer to an editor. Every novel you see get to print is really two. The book you write for love, and the one you re-write to sell. The best a writer can do is try to follow Owl's maxim, that "nothing be lost though everything be changed."