Bit of a late comer to this thread! Generally there may be many variables concerning the release of audiobooks. From my experience in the UK, Audible will have agreements with publishers to release audiobooks of the publishers titles. They will re release these titles already available in audiobook format via the publisher. In effect audible are a 3rd party seller in some instances. I do know that they will only release when five titles are made available.
Some UK audiobook publishers that release to libraries will not entertain an author unless they have a good library 'Presence'. That may well be changing given the current climate of cutbacks to library funding.
Some audiobook publishers may also only release one title in a series initially to test the the water. If sales go well then they will re visit the back catalogue and release along with future titles.
The audio rights must be bought by the publisher wishing to release and this can also affect what makes it to audio and what doesn't. Obviously the rights to a well known and popular author may be a lot more than those of somebody just breaking through.
The good news is that Audiobooks in the UK are growing in popularity and appear to be shedding the coat of being a product known more for providing a reading experience to the visually impaired. Hopefully now the number of titles available to able sighted and the visually impaired will increase. I did see mentioned in the news just last month that audiobook sales (along with ebook) had risen from £4million to around £16million in 2010. All good news.