Why do first ionisation energies decrease down a group?

This is an example of remembering the basic trends of ionisation energies of different elements across the periodic table and applying this knowledge in detailWe take the Bohr model of the atom where the energy levels of an atom are quantized and the electrons occupy a specific space in time.As we go down a group there are more shells, so more shielding ,so overall the attraction of outer electrons to the nucleus decreases.