In the publishing forum there have been questions about authors crossing between SF and fantasy, writing in both. The main answer was in summary, "it is pushing it to get out a good quality book once a year so publishers want someone who will deliver something saleable on time" get out as in written, edited etc. Hence publishers like you to stick to one genre. I would guess that applies to interrupting established series.
That said folks did notice some writers who were capable of putting out more than one book a year.
This thread has already covered all the different flavours of series and writers possible motivations and covered anything I'd say on that.
In terms of authors who want to air other projects but also want to keep series going, as a reader I'm noticing other options. "Mid-sized" press like Solaris here in the UK - I could be reading it wrong but I'm getting the impression that some of their works are from authors whose main publisher isn't interested in a certain book they've been offered but it is still of publishable quality and interest to the readers.
There is also self-publishing of unwanted by main publisher books. Diane Duane on her website has an ongoing project for a third book about the cat mages that she will be publishing through Lulu, that her main publisher didn't want as the sales on the first two were not good enough.
I'm really hoping as a reader that there is going to be more and more of this sort of thing. Shame that Meisha Merlin folded - I liked a lot of what they were putting out and hadn't finished buying what I fancied from them.
