The answer to 'why is my vacuum water damage' is that your vacuum cleaner has likely been exposed to water or other liquids, causing internal damage. This can happen if you've used the vacuum to clean up a spill, if the vacuum was left out in the rain, or if it was submerged in water.
Some common symptoms of a water-damaged vacuum include the vacuum not turning on, strange noises coming from the motor, or water leaking from the vacuum. In severe cases, the internal components may be corroded or damaged beyond repair.