I suspect our environmental laws affected the ingredients for printing books.
Inks that I used in high school print sop are totally banned.
Paper costs.
Freight at all stages.
charge cards (you don't really think those fees are paid by the merchant in the end

)
OTOH
Direct to plate technology helped keep pre-press costs down. (remember "Ortho " film

, Vari-Typer (tm), Linotype(tm) )