Exact reasons likely can’t be made public due to assorted legalities, however the most likely reasons certain books are passed on (be it permanently or temporarily) for digitization usually are:
-The archive copy/copies of certain books may be in poor condition and it may take time/effort/money to procure archivable copies that can then be scanned for high resolution digital purposes (this mostly applies to Golden and Silver Age books, especially for titles not originally published by DC, but it’s minutely possible early Bronze Age books could be applicable too).
-Some books may not be in their proper archive location due to bad records from years past, things getting jostled around and not being re-itemized afterward, etc.
From what I’ve seen of the DC archives on TV, in books, online, etc they look immaculate and then some, however mistakes happen and a book just might get put in the wrong series/area, which then leads to it not being easily found for later use. We’ve all mistakenly misfiled a book before I’m sure, so it’s likely archive staff may have too in very, very rare instances.
-There may be outside comic content (movies, TV shows, video games) that results in select titles taking digital prioritization over others. A series that has a good digital run so far may have to wait a while to continue, as the focus of the digitization team then shifts to books that will be digitized as tie-ins to outside media.
-Certain titles just aren’t a priority compared to others. It’s possible a book may go up to a point where someone then says “We’re stopping work on Title X so Title Y can be worked on because of…”. Basically, my previous point, but not necessarily due to outside media influence(s) either.
-Creator-owned works may get caught up in legal affairs, which can absolutely prevent series from digitally continuing or even existing in the digital landscape at all. This is said to be why we’ve yet to see any of the classic Milestone series make the digital transition.
There are other reasons books may be skipped over, series may stop at a certain point before they’re complete and the like, but the reasons above have most often applied to why gaps in existing digital series occur, as well as to why some titles/issues retain print-only availability.