Why isn't fat carried into the small intesting by the portal vein?

Anatomy. Simple answer - the portal venous system carries blood away from the intestines to the liver (where nutrients are metabolized).
Fat's in portal vein. The fat particles digested and absorbed from the small intestine are indeed carried by the hepatic portal vein, from the small intestine to the liver, for processing into the types of fatty "lipids" the human body needs. Over the thousands of years that humans roamed the earth in search for food, they evolved this efficient system for removing the maximum amount of nutrients from the intestines.