In a headline on their page for the books via the Thomas The Tank Engine section, an explanation is made due to the low stock of the books that they currently have.
Many are currently out of print, hence our low number of this range. We cannot get more new titles until the publisher does - reprint expected Feb 2012.
The last title to be released was The Spring Suprise earlier this year, an adaptation of the TV Series episode, Thomas and the Pigs.
Thanks to Jim522 for the alert.