Presumably because that is what it is set at by default. But when you're in the news feed section you can always hit Menu and select update to force a manual update.
If you want the app to automatically update more frequently go into the screen with all of your FB function icons, i.e. news feed, friends, chat, etc, then hit Menu and select Settings. There you'll see one of the options is the update interval. You'll probably see it set to 1 hr but you can change it to as little as 30 mins if you wish. But remember that a more frequent update interval will have a hit on your battery.
