Once you’re lucky enough for an Acquisitions Editor to solicit your book proposal, if at any point he/she becomes bored or finds you haven’t perfectly self-edited* your manuscript so it’s nearly flawless, the slush pile opens wide its mouth. If he deems your idea might be marketable, he’ll ask for the first two or three chapters, promising to respond anywhere from 3-9 months.
Then, if he likes what he reads, he’ll need to sell your idea to a committee of people. It’s possible they would all love your writing and think you are incredibly gifted, but you won’t be offered a contract unless the book can bring a profit. If the committee votes for your project, the production timeline is usually about 18 months, which comes after the nine months it took for you to get an answer. During the two years or so your manuscript may be tied up with one publisher, it is bad form to submit it to another publisher. And so you must wait.
After my first publisher rejected me, I followed up my other three remotely possible leads. When none of them panned out, the subsidy idea with Wine Press came about. Traditional royalty publishers pay the production costs, own the rights, and make the profit. With subsidy publishers, the author pays the production costs, owns the rights (meaning I can carve up my book and sell stories from it separately), and earns the royalties. It made much more sense for me to publish this way, and besides, it was my only option to have We Wait You in print. (By the way, the printing presses have started rolling!)
*For self-editing help, read The First Five Pages by Noah Lukeman and Self-Editing for Fiction Writers (even if you write non-fiction) by Browne and King.