The answer to 'why is my vacuum moldy' is that mold can grow in a vacuum cleaner due to moisture, dirt, and lack of proper cleaning and maintenance. Mold thrives in damp, dark environments, and a vacuum can provide the perfect conditions if not properly cared for.
Some common causes of mold in vacuums include leaving the vacuum stored with a dirty bag or canister, using the vacuum in damp environments, or failing to clean the filters and other components regularly. Mold can also grow if the vacuum is not dried thoroughly after washing or if it's stored in a humid area.