What is the reason that some of the most popular multivitamins are iron-free?

Iron harmful unless. Iron is an essential nutrient but most people get enough in their diet. The main need for iron is in menstruating women. For men and women past menopause, taking iron seems to do more harm than good, as excess iron is a pro-oxidant that can increase risk of heart disease & possibly cancer. See http://www.Healthy.Net/scr/column.Aspx?Id=68.
Safety. Multivitamin tablets are among the commonest item ingested, often overdosed, by the toddlers. Overdose of mutivitamin, in general, is free of harm unless taken way excessive, whereas overdose of iron is not.