This is pretty common for older series. There are a myriad of reasons; anything from ebooks not being covered by the authors contract with the publisher to rights for the cover art. The publishers will make them available if they believe there will be a market and as they get the rights situation ironed out. You can always click the link on the books page to ask the publisher to make the book available as a nookbook to show there is interest and if you find it available as an ebook someplace else posting in these forums used to help but now with the possible lack of a moderator I'm not sure that will be true anymore but it can't hurt. (search the forums, I believe there is a post pertaining to this; something like'Kindle book not available at B&N' or something like that.
There are a few sites I believe you can look for other sources for ebooks, inkmesh may be one. You can buy for the nook from just about anyone except Amazon.