In the US, midlist and backlist publications were negatively affected by the US Supreme Court decision in the 1979 case Thor Power Tool Company v. Commissioner of Internal Revenue.
Because stocks of unsold books could no longer be written down without proof of value, it became more efficient tax-wise for companies to simply destroy inventory.
The Thor decision caused publishers and booksellers to be much quicker to destroy stocks of poorly-selling books in order to realize a taxable loss.
[1] This has been somewhat mitigated by the development of online bookselling, which makes less popular titles more accessible to average readers.
This results in title proliferation, which itself promotes both lower advance orders on the part of major buyers, and a higher return rate.