IIRC, some of the electronic files of older Pocket novels were damaged/lost/incompatible with modern eBook-formatting programs
Not too mention that a lot of those early books (like Star Trek The Motion Picture & The Entropy Effect and even the original version of Probe as per Voyages of the Imagination) were written on typewriter, and as such there would probably be no original digital file of the manuscript, and if they put it out in ebook, someone would have to re-type or scan in the whole book.
I've got the ebook version of The Hardy Boys Casefiles #89 Darkness Falls and it looks like someone just ripped apart a printed copy and scanned in each and every page into Adobe and just created a PDF of the book that way.
But getting back to the original post, I think the original three books of the Destiny trilogy will probably fall into the out-of-print listings as the remaining stock runs out, and Simon & Schuster probably won't authorize any more printings since they have the omnibus edition out and that will be the "Destiny" book.