Only because of the scarcity of purple dyes. It isn't not allowed, just expensive to produce kinda.
Germania chose the color purple because it represents the power and ambition of the Germanian people, as well as to differentiate themselves from any past German flag.
Purple stands for royalty, that's why its such a more rich color
You wont see someone on the street walking around in a purple Shaw in London, 1722
You will see a queen, or princess wearing a purple dress in London, 1772
Because the real way the color purple was collected came from a certain animal that was very few in numbers and produced very little purple, so it was a very expensive color to produce and wear