If you've just signed a contract with an author, and have an option on their next book, would you prefer to see how the first book does, before you see their next one, or does it not matter?
Scenario 1: If the first book is a quiet one (ie, has a subtle hook), and the editor is feeling like "I love this, but will other people? I sure hope so," then she and everyone else at the publishing company are going to be a bit nervous about signing the author up for something else before they see how the first one pans out. I mean, if it tanks, then the buyers at bookstores will look at our rep and say, "I had to return our entire stock of the first one. And you're publishing another?" That's bad.
It doesn't change the fact that the quiet books are still worth making, but they can be a gamble. And we kind of hate gambling with $60,000 worth of other people's money.
Scenario 2: What you've proposed as a second book is a sequel to the first or a companion title in some way. They're similar enough for people to predictably link them. Unless the sales force is already swooning at the idea of selling the first one, chances are the publisher will need to wait and see how the first book is received.
Scenario 3: The first book has a reliable hook. It may not be a best-seller, but everyone's confident it'll pull its weight. And the second book you propose is not a sequel or companion title and also has a reliable hook. In this case, the publisher would be comfortable signing a second up before they've tested the waters with the first one.
We spend a lot of time thinking about the industry—what works, what doesn't—and we're pretty good at guessing. But every year there are bad guesses. And when you think that you could buy a house somewhere with one of your bad guesses, you can get pretty queasy.