Maintaining a digital archive through changing technologies can become a massive and massively expensive undertaking when the scale is large.
Making copies of every digital film to new kinds of media every 5 - 10, maybe 15 years will cost owners of those archives 10's of thousands, and more likely 100's of thousands of $'s, Pounds or Euros.
Certainly, maintaining a film based archive isn't cheap either, but it's not subject to quite the same technology churn that digital technology is.