Cost cutting, the advent of digital distribution, as well as technological and game design improvements that allowed for such information to be conveyed in game, all contributed to game manuals becoming less and less valuable. Due to these factors, they kept getting smaller and smaller until one day the just stopped being commonly made.

Though my nostalgia still misses them (and strategy guides).

I remember going to Toys R Us with Dad, picking a game out, and then eagerly reading the game manual on the long drive home. It got me SO hyped!! Definitely wish they still made manuals!