One reason could be that while new books are often done from a reasonably digital source, older ones often have to have said source made either by scanning or otherwise re-creating it.
One of my favourite fantasy/historical/mystery authors, Barbara Hambly, has been trying to digitize her older out-of-print rights-reverted books, but apparently her work team consists of her mother typing it all manually from paper copies after she lost her original manuscripts to a hard drive crash years ago.
If an author/publisher has to do it this way and then make an e-book format on top of that, then it may be too much of an added cost for whomever the current rights-holder is to consider it worth their time, especially if the book is still in physical print and/or seemingly low demand.
Sf author Norman Spinrad has reportedly been cleaning up darknet copies of his older books in order to make the legit versions which he sells in the Kindle store, so that's a route that could be taken in order to cut down costs/time.
But this probably works best with genre fiction (especially speculative fiction), which tends to have a devoted following willing to take the time to scan/convert/upload relatively obscure stuff for completion's sake and also have the technical know-how to begin with.
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