The answer to 'why is my steam cleaner moldy' is that mold and mildew can build up in the water tank, hoses, and other parts of a steam cleaner if it's not properly cleaned and maintained. This can happen if the steam cleaner is stored with water left inside, or if it's not thoroughly dried after each use.
Mold and mildew growth in a steam cleaner can lead to unpleasant odors, and can even spread to the surfaces you're trying to clean. It's important to empty the water tank, clean all removable parts, and dry the steam cleaner completely after each use to prevent mold and mildew buildup.