On my travels I have been reading the weighty Books in the digital age by sociologist John B Thompson. He discusses the impact of library budgets on the publication of scholarly monographs in several places.
The most important fact in explaining the decline in the sales of scholarly monographs is to be found in the financial pressures faced by the research libraries at the major universities. The growth of academic publishing in the 1950s and 1960s was fuelled by the expansion of universities in the United States, Britain and elsewhere…. [page 98]
He notes the squeeze on library budgets beginning in the seventies, the growing importance of journals, and the rise of digital formats as factors here.