The answer would be that the Electrons get farther from the nucleus. As you go down a group, the Period is increasing, which means that each time a shell is being added on as such the valence electron are moving further away from the nucleus. It is therefore easier for the valence electron to be removed thus why less energy is needed to remove that electron, thus a decrease in ionization energy.
It was probably because there was no organization to the order of the elements so they got frustrated since they didn't know where things went.