(via Richard Charkin blog) Richard Charkin is the Chief Executive of Macmillan (Nature Publishing is a subsidiary of Macmillan). He posted his thoughts on digital books are not as successful as the digital publishing going on at Nature.
I can't help noticing the second reason (my emphasis):
2. Scientific publishing has been intrinsically more profitable than trade book publishing. This allowed the major publishers and societies to invest the significant sums needed to create electronic delivery and storage platforms for scientific information. These platforms are a cornerstone for the creation of a new business and communication model.
and read it as "higher profit margins".