Historically, purple dye was so expensive that it was only used for the personal vestments of the emperor (in the Roman Empire, “the purple” was the standard synonym for the office of emperor, in the same way that “the throne” or “the crown” is used in modern monarchies).

It could only be obtained by harvesting a particular type of shellfish in Phoenicia or Mauritania, and obtaining enough of it for bulk-produced items like flags would have been unthinkable.

You should also ask this over in /r/vexillology

You might want to cross post to /r/vexillology/