There are several reasons. First, if our series processing inventories indicate that we still need the report, we actively seek the report.  We do not have the staff to check our needed items against what was digitized and made available in HathiTrust by another content provider.  Second, ONLY reports digitized by TRAIL are indexed and searchable by our search interface.  Finally, technical reports that have maps, foldouts, etc. are rarely included in the reports available at HathiTrust.  TRAIL sends these reports to the University of North Texas (UNT), our other TRAIL repository, where the maps, foldouts, etc., are digitized along with the reports.