Elements in the same group (vertical column) of the periodic table have the same number of valence electrons. As you move down the group, the number of electron shells increases.

1) The valence electrons are farther from the nucleus.

This means that the electrons are less strongly attracted to the nucleus and are therefore easier to remove.

2) There are more core electrons.

These core electrons shield the valence electrons from the nucleus, reducing the effective nuclear charge experienced by the valence electrons. This also makes the valence electrons easier to remove.

The ionization energy is the energy required to remove the most loosely held electron from an atom. The higher the ionization energy, the more difficult it is to remove an electron from the atom.

Therefore, elements at the top of the periodic table have higher ionization energies than elements at the bottom of the group because the valence electrons are closer to the nucleus and the effective nuclear charge is greater.