For old series, they may not have the rights to publish the eBook or have yet to get around to converting them. But I agree it's very annoying.
Fortunately it's the kind of issue that will go away with time, I wouldn't be surprised if most authors push to get their entire back catalogs available for sale, it's one of the advantages of eBooks
Best you can do in the mean time is email the publisher/author to see if/when it'll ever be released.