It's simply a fact that most textbooks now being published were unnecessary. The textbook publishing industry exists primarily to shuffle existing material to produce new editions on an almost annual basis (to destroy the used book market), and to produce custom editions organized according to the whims of state education boards.
The need for new material in something like a math book is truly rare. In the past twenty years, we've developed new instructional technologies, but a textbook really shouldn't be tied to a specific brand of calculator or programming language. I'm currently teaching precalculus using a 1994 edition. It's got some exercises to be done using a computer, and has some sample code in BASIC, but they're easily adapted to modern tools. I'm also making use of a textbook published in 1967, because it does a better job at explaining certain topics. Sure, the word problems have prices in cents instead of dollars, but that doesn't make it any less effective.
I'm using those old books because they are the best books I have available. I have newer books available, but they eschew clarity for colorfulness and newer "teaching practices" that only get in the way of actually teaching.
The need for new material in something like a math book is truly rare. In the past twenty years, we've developed new instructional technologies, but a textbook really shouldn't be tied to a specific brand of calculator or programming language. I'm currently teaching precalculus using a 1994 edition. It's got some exercises to be done using a computer, and has some sample code in BASIC, but they're easily adapted to modern tools. I'm also making use of a textbook published in 1967, because it does a better job at explaining certain topics. Sure, the word problems have prices in cents instead of dollars, but that doesn't make it any less effective.
I'm using those old books because they are the best books I have available. I have newer books available, but they eschew clarity for colorfulness and newer "teaching practices" that only get in the way of actually teaching.