Why is a fat diet not good for you?

Moderation. How much fat are we talking about? You need carbohydrates- to run the brain, heart, muscles etc. If your diet is over 50% fat, you may be limiting the ability of the body to function. Some fats, like unsaturated fats are healthier than saturated fats- are solid at room temperature. I say moderation- 20% of diet from protein, 60% from carbs like fruits, vegetables, grains and 20% healthy fats.
May not be good. Fat is an essential human body component. So, our diets invariably contain some fat. "excessive" (the norms differ for different people based on their health status) dietary fat may lead to untoward consequences such as atherosclerosis (fat build up in the arterial walls) leading to ischemic heart disease that may result in heart attack, stroke etc.