Perhaps there's a selection effect where people who buy electric toothbrushes are pre-disposed to care more about dental health.
> 5068 participants were randomised to receive either a powered toothbrush or a manual toothbrush. Majority of the studies included adults, and over 50% of the studies used a type of powered toothbrush
https://www.cochrane.org/evidence/CD002281_poweredelectric-t...