If you still lived in the early days of printing then this question would have never crossed your mind because two color printing (or offset printing) was the only option available in those times.
No matter how good the book is, it is very difficult to estimate the number of copies that will sell. The recent economic slowdown where like everything else book sales slowed down made the authors and publishers re-strategize and look for a solution.
Self-publishing is becoming very popular these days. Many people are opting for this to keep their creative freedom intact which gets compromised at times with the publishers.
Even though digital printing has been in vogue for quite some time, it is still a new technology that many people are not yet used to. It is definitely much faster and reliable that the traditional methods of printing.
