I have distinct memories of the days when the prospect of digital media everywhere led to thoughts of how easy it would be to preserve everything: Digital Will Never Die! The basic idea was that, as digital is All or Nothing, the signal is strong and clear and so resistant to degradation. All we have to do is just keep transferring it from one substrate to another as the substrates wear out.
Piece of cake.
That's not how things have worked out. David Bordwell has written a useful essay on the nasty problems of digital preservation: Pandora’s digital box: Pix and pixels. The law of unintended consequences strikes again, and again; as Bordwell observes: "It seems likely that digital projection has, in unintended and unexpected ways, put the history of film in jeopardy." There are many problems, more than I care even to list, much less summarize. Let one little paragraph stand for many:
Storing 4K digital masters costs about 11 times as much as storing a film master. You can store the digital master for about $12,000 per year, while the film master averages about $1,100.
For that's what it all comes down to, cost.