Yeah, it's great information for the new DMs, but how many new DMs sit down and read those sections off the bat? My guess is a good many struggle just to learn the basic rules and turn to those sections down the road when they have the time or when they're trying to figure out why their game is going south.
And I don't know about others, but after nearly 30 years of playing this game, sitting down and reading that stuff isn't something I come into a new version to do. Between 5 distinct editions of D&D, Dungeoncraft articles in Dungeon and Out on a Limb articles in Dragon I'm sick of reading the same ol' stuff. So while you can fill the DMG with information for newcomers, to us veterans it's old hat information so don't devote too much to it.