The reason was because people or rather to say the majority of general consumers does not bother reading manuals or instruction booklets anymore nowadays much more even touching it. The industry was made very well aware of this and thus software companies these days omitted it altogether or just provide a small thin sheet or a digital manual altogether.
Other than that another obvious advantage for the publisher and game manufacturer is that it saves on printing cost.
I remember when I was a kid I wasn't allowed to have many video games (had pokemon yellow and RE 1, Chinese bootleg ftw) so I would just go online and read walkthroughs to experience the latest REs. Skip ahead 12 years when I do have a disposable income to buy w/e the **** I want whenever I want RE goes to complete ****.