The theory I've encountered is that fluoridation previously had an impact on cavity-prevention before people were actually brushing their teeth with fluoridated toothpaste. Now that toothbrushing is much more common, the impact has waned.
Late edit;
Here's a look at toothbrushing frequency in Europe by country from 1994 - 2010:
Just imagine how much improvement you'd see from if the chart started in the 1940's and 1950's when many cities began fluoridating their water supplies. Not to mention that many early toothpastes were often ineffective at preventing cavities by primarily relying on baking soda and peroxide.
remember that non controlled measurements are worthless.
same time they started water fluoridation the population also started to pay attention to brushing their teeth. or do you think the govt took the initiative at any point in history?
Late edit;
Here's a look at toothbrushing frequency in Europe by country from 1994 - 2010:
http://i.imgur.com/VVBldAy.jpg
Just imagine how much improvement you'd see from if the chart started in the 1940's and 1950's when many cities began fluoridating their water supplies. Not to mention that many early toothpastes were often ineffective at preventing cavities by primarily relying on baking soda and peroxide.
(Full brushing frequency study: http://eurpub.oxfordjournals.org/content/25/suppl_2/20 )