The same things seems to be happening with movies as well. A key but subtle point is that sequels these days are far more often planned from the beginning, rather than being set in motion after a film does well. It's changed from "if the movie very well, maybe we'll do a sequel" to "we'll do a sequel unless the movie tanks".
I wonder if it is because of a demand for more richly imagined worlds. The more detailed the world, the more effort to create it and the more incentive to use it again. Of course, the Lord of the Rings is an early example, although the re-use was rather limited.
If that's true, there a similarity to the world of software where nowadays everything is a sequel. This is because a good application is the equivalent of a fictional world (I like to call this the "conceptual space" of the application). The shape and structure of this world generates the "feel" of the application in the same way a fictional world generates the "feel" of a movie or book. In the early days of computers there wasn't much to these worlds so it wasn't a big deal to create a brand new one. Now, however, there are clear genres of software and common conventions that are expected. These must be provided or the application is doomed, but each of these adds additoinal cost and effort to shippping something. So it's a lot easier to make sequels because you've already done most of that work.
Is there something else driving this, that we're seeing it in multiple fields? For software it may just be a maturation process, but for movies and books? On the other hand, the completely imagined world is a relatively recent invention and so it may be finally maturing as well (on book time, not Internet time). Perhaps it lowers the transaction costs for both producers and consumers. Finding good books or movies or software is becoming ever more difficult because of the ever widening array of choices. Branding serves to provide a good but not optimal result where the benefit of a truly optimal choice is less than the costs of determing that choice.
As a final observation, this is part of the growth of the "super-star" economy, where the best crowd out the good (evolving toward the 80/20 rule). This has its good and bad aspects, where it raises the overall qualiy but tends to stifle innovation. It seems to be an inevitable result of an open, technological society so we'll just have to learn to adapt.