The answer to 'why is my vacuum mildew' is that mold and mildew can build up in your vacuum cleaner, especially if you don't clean it regularly or store it in a damp environment. Mildew can cause your vacuum to have a musty odor and can even affect the performance of your vacuum.
Some common causes of mildew in vacuums include leaving the vacuum bag or canister full for too long, storing the vacuum in a humid area, or not cleaning the filters and other components regularly. Symptoms of mildew may include a foul smell, reduced suction power, and visible mold growth inside the vacuum.