I think Zapdos fell significantly because it just cant take hits as well, and when it does take resisted hits its usually a U-turn or Volt Switch which wears it down with SR.
It almost useless against stall, KOed by all Sun Sweepers, cant take a hit in the rain, loses to most Pokemon on standard teams (Terrakion, Ttar, Land, Latis, etc).
Zapdos was suppose to keep Lucario, Metagross, Suicune, Vaporeon, Celebi, Breloom, Shaymin, Scizor, Togekiss, etc in check. Most of these Pokemon are very rare now in OU, and the most common of the Pokemon listed can U-turn or Breloom KOes with +2 Mach Punch after SR.