Well to answer this question we have to take a trip down through history. To put it simply prior to the mid 1800s purple dye was simply too expensive to purchase for most people and as such purple clothes were only found on the backs of the extremely rich and royal people.
The purple dye was gotten from a snail which was only found in the Venetian trading city of Tyre which today is now located in modern day Lebanon. The dye required about a 1000 snails to produce just 1gram of it and as of then it was so expensive that it became a symbol of royalty.
At a point even the royals couldn't afford it as it became more than worth its weight in gold. As of then a pound of that dye cost about 3 pounds of gold which is worth about 56,000 dollars right now...imagine having 56,000dollars worth of steempower.
At that point, no country could afford having purple on their flags, not even the richest countries in the world.
It wasn't until a certain chemist by the name of William Henry Perkin accidentally created a synthetic purple compound while attempting to create a malaria drug. Once he realised that he could use it to dye clothes purple, he patented his discovery and became super rich.
After that purple became more Accessible to everyone but it still wasn't a norm to use it in national flags.