I can affirm, because I asked, that the Pangborn books were withdrawn from e-reads by the estate. The truth is that most backlist books sell very poorly in ebook format unless the author is either writing new books or out there actively promoting the older books. A few hundred dollars a month for an old title would be considered a great success. It may be that poor sales were the reason for the withdrawal of the Pangborn books.
I suppose it's also possible that Beagle either dislikes ebooks or has other ideas about how to proceed with them.
For us readers it's a shame, with respect to both authors.