Why don't country flags use the color purple?

Purple can have connotations of the nobility or royalty, and also it can be associated with the supernatural or magic. Most likely countries and their people don't feel that the color identifies with them. Also as M.Montt suggested purple historically was a very expensive dye to obtain due to its scarcity in nature.    

i hope this helps and have a wonderful day!!

Fine until slavery ya ya haha