The answer to 'why is my vacuum mildew' is that mildew can build up in a vacuum cleaner due to moisture, dirt, and debris getting trapped inside. This is a common issue that can happen over time, especially if the vacuum is not properly maintained and cleaned regularly.
Some common causes of mildew in vacuums include using the vacuum in damp environments, not emptying the dust cup or bag frequently enough, and failing to clean the filters and other internal components. Mildew can also form if the vacuum is stored in a humid area or not properly dried after use.