I am curious, and you don't have to answer if you're not comfortable. Was your advance for all three books pretty good?
If I was a debut author ,I would probably hold off on a trilogy for monetary reasons as well.
If my first book is going to get me a 5K advance, I would want to see what the response was to that book. If there was an success to the first book, I would assume the next book would be worth quite a bit more.
If I sold my first three books at the same time, I'd expect a 15K advance, which might seem paltry in the grand scheme of things.
Plus I'd want to stagger the release.
It was six figures (total, for all three books). Publisher's Marketplace noted it as a "very good deal."
There are a few things to remember though. Big advances are awesome, but if the book doesn't earn out or make the publisher money, it can kill a career. A smaller advance can be good too, as there's less risk that you'll be a flop. There are pros and cons to both sides.
You also keep getting royalties on your book once you earn out your advance. So if your book does well, you make the same money, just on the back end instead of the front. And you're in a better negotiating position for your next book.
Example:
For easy math, lets say your royalties are $1.00 per book.
Book one gets a $5K advance, with a 10K book print run. It sells 9,000 books. You earn out your $5K advance (they publisher keeps the first $5K), then make $4K royalties. That's a huge sell through (the amount of books you sell at a given store.)
Book two gets a $100K advance, and a print run of 75K books. It sells 20K. You get your advance, but no additional royalties and the publisher is out money. The book is a flop.
When both these authors send it their next book, author one is a much better prospect than author two, even though author two sold more books. But they under performed as expected, so it's a loss, even though they sold more books.
About the only thing you have control over as an author is how well you write your book. Once it's sold, the publisher has control, and they'll do what they think will be best for the book. They pick the release (mine are in the fall for 2009, 2010, and 2011), they choose the cover, they might even change the title. (they did mine).
There are so many variables and so much of it is out of both the author's and publisher's hands. In the end, it's the consumer who has all the power. If they love a book they'll buy it. If not, they won't. Publishing just works to keep up with what the consumer does.
It's a crazy business, but I think all creative industries are

That fine line between genius and insanity I think. .