There are a few reasons why Politoed dropped to Untiered in XY, one of the highest drops ever.
First, with the end to the Weather Wars of Gen 5 and the huge nerf of weather in Gen 6, Weather teams started becoming more and more uncommon, which led to a lower usage rate for Politoed. So, Politoed only had a staple on Weather Teams, which were far less common due to the Weather nerf.
Second, Drizzle was banned in UU and below. This led to Politoed's only usefulness to be way down in the tier of PU as a Water Absorber and Encorer. However, even then, it was destined to suffer there with poor Physical Bulk and Speed which leads it to be ripped apart by those Physical Wallbreakers in the tier.
These are the 2 major factors that caused Politoed's historic drop, the huge need to weather and the banning of Drizzle in UU and below killed Politoed.