The in-game tutorial became standardised and ultimately makes more sense from a game design perspective. If something is interactive, why not present a tutorial to the player interactively, rather than through a separate out-of-game booklet?
Producing an instruction book is ultimately a cost. It costs money to make them and their value was continuing to diminish from a design perspective, which led to them being made cheaper (no colour, fewer pages) which in turn led to their minimal artistic value being gone too.
The artistic value they once had can now be provided through a more limited and cheaper release that doesn't waste resources if people want - you want the official art? Get the game that comes bundled with an artbook.
>>724754065
Digital manuals are a worst of both worlds situation. They lack the physical appeal of owning a book that relates to the game you like, while also still not actually being in the game itself naturally, making them worse than an in-game tutorial.