Everything's published in threes now, especially high-concept books. If it's interesting enough for one book, than it can be sustained through a trilogy. It's understandable, really. Publishing is a tough business, and the market gets more competitive every day. So if a writer has a good idea and the talent to pull it off, a trilogy means three-times the sales and three-times the time to gain a loyal following. But in doing this, has the publishing industry created a new kind of reluctant reader?
Now you're expected to commit to an entire series--in essence a 1000-page book published over a three-year period--with no promise of a return on your investment. That's probably why so many readers are overly critical of the last book in the series. "I've spent countless hours and nearly $75 on these books, so the series darn well better end how I want it to. And if it doesn't exceed my expectations...well, then heaven help the author."
Gone are the days of the episodic series of self-contained books with a distinct beginning, middle and end. Now everything is epiphanic, where a single sentence in the second chapter becomes critical to the plot two books later. You see this a lot in modern television as well. If you miss one episode, you're completely lost. But it's also not a new story-telling device. Serial writers like Charles Dickens and Robert Louis Stevenson used to do it all the time on a much smaller scale with their magazine fiction.
I don't even think it's a bad device. If done well, it allows better character development and exciting plot twists. It heightens the suspense and makes you crave to see what happens next even if you aren't sure you'll like it. But sometimes I just want to sit down and read a good book without having to worry that it will be another two years before I can read the end.