the "thank you captain obvious" answer is that it's because the cells in the intestines don't have an iron 3+ channel. There are a lot of iron containing proteins used in cellular respiration, not to mention hemoglobin. All the iron you eat will be a mix of both oxidation states. In its reduced state, the body can incorporate it into proteins. If the iron was absorbed in the +3 state, the cells would have to reduce it in order to use it in synthesis which would waste precious electrons.