You could give targeted help to the ones that needs it, it would be more efficient to help them brush their teeth than just adding fluoride to the water that everyone drinks.
We do it by noticing who needs help on the free dental checkups during the first years of life usually. If they have some sort of handicap, they can get help physically brushing or support to do it themselves every day. Helps a lot more than adding something that might work a little in everyones water.
On what basis have you evaluated that it "helps a lot more"? What if your method doesn't lead to more people actively brushing their teeth, thus reducing the amount of people with properly protected teeth (since they are also missing fluoride now), increasing amount of dental healthcare required?