Developers can't offer free trials to their customers on the App Store, and customers aren't always willing to commit dollars to new apps. That's normal. Even though most apps cost less than a cup of coffee, at least a cup of coffee is something that people know and understand—everyone knows what a cup of coffee offers and can easily decide if it's worth the price.
Apps—especially new, unrated apps from lesser-known companies and developers—don't necessarily have that kind of obvious value. Sure, you can check the app's website, or watch a cool preview video on its App Store page, but most people just won't invest that kind of time and effort.
It's not a sustainable business model in any way, so it won't happen often (if ever), but by making apps free for a short period of time—in this case, one week—people get a chance to try them with no commitment.
It's the next best thing to a free trial.