The short answer is, because traditional authors are so hamstrung by their contracts that writing anything more than their permitted one book a year is a waste of time.
Of course, the reality is more nuanced than that.
I have been traditionally published and know that world a little.
On the other hand, I have been 100% indie published since March 2011, and I do know that world very well.
Indie fiction authors benefit from an accelerated publishing schedule (see “Why Productive?“).
While traditionally published authors may like the increased creativity that comes with a faster pace of writing (more on that in the coming weeks), their productivity challenges are far different from the average indie author.
As productivity for indie authors is already complex enough, I have deliberately narrowed down the scope of the topic to indies only.