tl;dr - When there is no incentive to make available what is owned, it is not made available. Copyright extensions have made most mid-20th Century content owned

tl;dr - When there is no incentive to make available what is owned, it is not made available. Copyright extensions have made most mid-20th Century content owned, but with little incentive to make it available. As ownership is revoked by expired copyright, others may own (non-exclusive) that content and thus have incentive to make it available in new production.