I receive a lot of queries and manuscript submissions. When I like what I see, I am always likely to ask what else the author has written.
Because I'm not interested in a one-hit wonder.
You see, with each book we publish, we have to establish a marketing plan around the book and around the author. We have to reach the audience most likely to buy the book, and we have to do that through radio, television, newspapers, magazines, social media, blog tours, book stores, libraries, and a host of personal appearances and events.
All of that takes a lot of time, effort and money. Sure, you could be the next Harper Lee or Margaret Mitchell. But chances are, you're not. Chances are, your debut book will sell decently or not at all. But your second book will probably sell a little better. And your third, better than the second. And by the fourth book, your readers are asking what else you wrote and looking at your backlist.
You can't grow your fans if all you have is one product, anymore than you can open a convenience store and sell one item.
There are the exceptions - but they are exceptions.
For the average author, you need a line of products - a line of books. You need to market and promote consistently and your readers will get tired of you hawking the same book and they'll turn a deaf ear. You have to stay current, just as musicians need to keep putting out albums and actors need to keep appearing in movies. Your chances of making it is less than one percent. Your chances of making it with one book is less than winning the lottery.
So while you're trying to find the perfect agent or the perfect publisher, keep writing. You'll be glad you did.