Fluorine is in a lower energy level wherein the electrons are closer to the nucleus, thus increasing the coulombic attraction. Also, Iodine has more shielding present due to the multiple energy levels between the nucleus and the valence electrons. Also, both fluorine and iodine are in same group, so their "effective nuclear
charge" is the same. But since the valence electrons in iodine are farther away from the
nucleus compared to the valence electrons in fluorine, it is easy to
remove electrons from iodine. That's the reason iodine has less
ionization energy compared to fluorine. This is true in every group : ionization energy decreases going down
