The government-approved version of history is explained to Montag by his boss, Beatty, when Montag becomes depressed at his job without understanding why. Beatty's explanation is that society became too large for books to have relevance; with every minority or group in society able to be offended by books, they became a source of contention and unhappiness instead of knowledge and entertainment. Because people liked the simplicity of television, books were deemed objects that promoted unnecessarily complicated thinking, so firemen were retooled to burn books and keep people stupider, but happy.
In reality, as the government gained more and more control over news and entertainment, they decided that books were a threat to their total control of societal opinion. Books are unregulated, and they allow any person to read ideas that oppose the official ideas. Since books threatened the government, it banned them, keeping control over the population with disposable, pointless, safe television programming.
Things became as they did in Fahrenheit 451 for a variety of reasons. One reason is that technology led to the development of fireproof building material so that firefighters were no longer needed for their original purpose. American society grew rich while the rest of the world suffered, and during this increase in wealth, people became less involved in thought and more involved in entertainment (the epitome of this is the wall screens).
Books represented free thought, and it was believed that free thought was dangerous, causing depression and a variety of other "unwanted" behaviors and emotions. Having no need to put out fires, the new role of firemen became to burn books. It was an act with the sole purpose of reminding people that free thought was "dangerous". Beatty, who had read many books as he felt was necessary for anyone of his position (and even seems to have loved books at some point in his life), believed that books are dangerous and in need of being destroyed because they manipulate the reader. Men who were long dead could still have influence over the populace through their words written in a book. One book could easily contradict another, and such influence lead to suffering. Possibly, Beatty antogonizes Montag because he is so unhappy that he actually wants to die, which could mean that despite his promotion of the dangers of books, he is still very unhappy in the new world that has been created.