When and why was the notation changed from descriptive to algebraic? When I was young I used to play chess with the the old form of notation KB (King Bishop square number), even books used to have the same notation, then later found that everyone was using algebraic, is descriptive notation dead? Why?
Though the previous answers have made some solid points, I think they have not yet given the main reason why algebraic notation has generally come to be preferred over descriptive notation: it is more absolute, less relative, than descriptive notation. What I mean is just this: in algebraic notation each square on the chessboard receives only one name, while in descriptive notation each square receives a different name depending on whether a move by white is being notated or a move by black. This alone makes algebraic notation more transparent, because things aren't relative to which side is on the move.

Consider, say, the g4 square (in algebraic notation). When white moves a piece there, descriptive notation says it is headed to KN4. But if black is moving a piece there, it is headed to KN5. There is just no need to have two names for this square, and it serves only to sow possible confusion in a way that algebraic notation does not.