Doctors are typically educated in how to fix problems, not so much in prevent them. This is unfortunate, since most people think they're experts in everything including nutrition, and treat them as such, but they're really not.
Maybe if doctors were paid to keep their patient base healthy rather than just fix them when they're sick, they'd look into more of that.
Capitation is one approach to solving that problem.
http://en.wikipedia.org/wiki/Capitation_%28healthcare%29
Unfortunately the times frames are still fairly short because patients often move around. So even when doctors are compensated through a capitation system, there isn't enough incentive to encourage patients to make healthy lifestyle choices that will only pay off many years in the future.
Maybe if doctors were paid to keep their patient base healthy rather than just fix them when they're sick, they'd look into more of that.