Frankly I enjoyed King of the Ring and it was responsible for propelling quite a few of the industry's greats including Steve Austin, Triple H, Owen Hart, ect. If went a little off-track during the Attitude era, but if the WWE cared enough about it, they definitely could have saved it. Even the "King Booker" angle was excellent. An annual tournament, regardless of it's physical reward, always has potential to add credibility to any wrestler.
The plain honest truth is, Money in the Bank replaced it. Wrestling fans today just don't have the patience to endure a month-long arduous tournament like King of the Ring, they want to see weapons, chairs, ladders, tables, and all sorts of other bullshit and get it out of the way quickly. MITB serves the purpose more quickly and more violently, involves just as many people, and gets the whole ordeal out of the way easier. In the modern WWE, you just can't have MITB and KOTR in the same world, and to most fans it seems cooler to have a guy carrying around a briefcase for a few months before he becomes champion as opposed to a guy wearing a crown and carrying a scepter instead. MORE AMERICAN! Ohhhh saaaaay caaaaan you seeeeee... by the daaaaaawn's eeeeeearly liiiiiiight...