The issue is that humans are biological creatures with brains made up of many different modules. Antonio Damasio wrote a book about this called Descartes' Error.

We are much more emotional than we are rational, and as we can see in Descartes's case, we mostly use reason to justify our emotional biases when we make decisions. There's a theory about why that is which may interest you.
https://sites.google.com/site/hugomercier/theargumentativetheoryofreasoning

If you're interested in large-scale social change, check out the public health work of Mexican TV producer Miguel Sabido, whose telenovela method has been used around the world to change people's behaviors.
http://www.apa.org/monitor/oct02/theory.aspx