Because before gen five, the only mon in ou that had access to a weather summoning ability was Tyranitar in gen three. In gen four, Hippowdon and Abomasnow joined in. Gen five changed that when Politoed and Ninetales got drizzle and drought, which allowed them to contest Tyranitar's weather. Before gen five, neither of those two had the weather abilities. It is also notable that the only way to abuse hail was with snow cloak and with Walrein's ability. Snow cloak got banned in gen five along with sand veil when smogon finally decided evasion was bs and Walrein was well, it sucks
It is also worth mentioning that weather wars did exist before gen five but not in ou. It happened on ubers where Kyogre and Groudon were driving Rayquaza insane
Forgot to mention. After gen five, ou was no longer infested with weather because they no longer last until the battle ends. This allowed for various styles of teams that don't necessarily need to run a weather just so they don't lose to another weather. Ubers also no longer has a weather war because of how the primal's abilities work and no sane person would not primal revert their Groudon or Kyogre
