1) Diet and fitness are hard problems because humans evolved as opportunistic hunter-gatherer-scavengers, so we are moderately well adapted to almost any imaginable lifestyle. When the optimum is broad and shallow (which it necessarily is, especially for diet, unless you are an evolution denialist) it is easy to wander around in the noise.
This is made worse by snake-oil salespeople who are dedicated to the idea that the optimum is narrow and deep, and they can sell you its precise location. They take any minor wobble that scientists identify--which based on evolution is almost certainly noise--and declare it the One True Location of Perfect Health.
2) Science fails to get traction with the public because it lacks narrative, which is an idea I explore in a lot more depth here: http://www.amazon.com/Darwins-Theorem-TJ-Radcliffe-ebook/dp/...