Probably a lot of gunk has accumulated. A moist area like that is a great place for various kinds of smelly things to grow. The way to deal with this is to clean out the drain with some kind of drain cleaner. Usually you let that stuff sit in the drain for awhile then flush it out with water.
Two things come to mind. First if the trap in the drain becomes empty maybe due to a leak or evaporation, this would allow sewer gasses to come up. And second if anyone in the household uses a Dandruff shampoo that contains sulfur this will smell if not rinsed away properly.