In Iran, the milk in the city I'm from was all procured by non-jews. So unless you bought a whole cow for pesah, you wouldn't have dairy because there was no means of verifying if there was hametz in the milk.
At what point in Jewish history did waiting 6 hours between milk and meat become the uniform approach in most communities (or in some places 1 hour or 3 hours)?
According to this article it was the Rif (Rabbi Yitzchak Alfasi 1013-1103) who instituted the fixed waiting time.
Read the article for sources and background.