HathiTrust aims to have a comprehensive representation of the published record, beginning with items digitized from member library collections. There may be items that are prioritized by member libraries (for instance, because they are technically easier to bring in), but our current limitations are really the scope of what has been digitized and our ability to bring it into the repository cost-effectively.
There could be a number of reasons why something has not yet been brought into the repository. Examples include: the physical volume is in poor condition and is not suitable for scanning, the volume has been scanned but the quality of the files is poor, the publisher opted out of scanning the volume, or the volume is not owned by a HathiTrust member library.
You can find out more about our collection development principles here: https://www.hathitrust.org/collection-principles
\uD83D\uDCD8 When you are interested in an item that is missing from the collection
It is sometimes difficult for us to know whether the title you are interested in will be added to the HathiTrust collection. HathiTrust itself does not do any digitization. Instead, our member libraries decide what they want to digitize from their print collections and whether to add those books to HathiTrust.
You are welcome to reach out to member libraries that have a physical copy of the book or volume, and ask if they can scan and submit it to the HathiTrust collection. You can see a list of our member libraries here:
https://www.hathitrust.org/member-libraries/member-list/