Where in "Fahrenheit 451" does Bradbury say that the question of why is not being used or is fading?
The passage that you are looking for is probably in the section where Beatty comes to Montag's house and gives him a brief history lesson of how it came to be that books were being burnt. In this lesson, he describes how people became less and less interested in real thinking, because it took too long. People wanted to know what a book was about, without having to read it themselves. That way, they could feel smart without having to go to a lot of effort. In a nutshell, people were lazy. Now, the government were just peachy with this tendency that people had, because if people were too lazy to read, that means that they were too lazy to think for themselves. It is independent, free-thinking people that typically cause problems for government--they are the ones that inspire change, rebellion, revolution, and dissatisfaction on a mass scale, that ultimately takes the power away from the government. So, their society encouraged such lazy behavior; they encouraged books not being read; they enouraged people not thinking. However, they wanted people to feel like they were thinking, to feel like they were learning, so that their minds weren't bored or restless. So, instead of giving them deep, profound philosophy that might inspire independence, they gave them shallow, fake learning instead, to give them the feeling of learning without learning anything "dangerous." Beatty states it as follows, in speaking of Clarisse and her family:
"She didn't want to know how a thing was done, but why. That can be embarrassing. You ask why to a lot of things and you wind up very unhappy indeed, if you keep at it."
So, he states later, they try to "nip most of them in the bud," meaning, the people that ask "why" too often. Instead, they keep people busy with television, activities, fast cars, and useless information; that way, people don't ever get curious because they're so busy being entertained. I hope that was the passage that you were looking for; good luck!