The answer is simple. With Facebook Credits, games don't have the ability to give them out for free. They can't offer them randomly as quest rewards, or start you out with any for free. It's basically a currency that the developer has no control of. Now that Nightclub City has added an intermediary currency, they can give them out to players in promotions or just for fun. This will be a benefit to you as a player!
For anyone who is confused, Nightclub City released an FAQ guide that helps players understand the change. The developers have stated that the prices for items should be exactly the same as before, except they offer bonuses for large purchases so it might even be cheaper. We definitely like this change!