Japan is a crowded country, and that goes for just about everything. Even store shelves are crowded. Most people do their shopping on foot, which means supermarkets and convenience stores tend to be on the small size, and shelf space is always at a premium.
As such, companies have to do something to make their products stand out. This is why so many Japanese beverages and snack foods have seasonal flavors that are only available for a limited time. Of course, taste engineers in Japan can only come up with so many normal flavors, and when they run out, the only thing to do is go on to the abnormal flavors.