Why do Permian, Ostyak and Oirat require Royal Court to have Tribes of the North?

[Slime99]

oirat = {
    color = { 0.2 0.2 1.0 }
    
    ethos = ethos_bellicose
    heritage = heritage_mongolic
    language = language_mongolic
    martial_custom = martial_custom_male_only
    head_determination = head_determination_herd
    traditions = {
        #tradition_horse_lords
        #tradition_loyal_soldiers
        tradition_hunters
        tradition_zealous_people
        tradition_mountain_homes
    }


    dlc_tradition = {
        trait = tradition_mpo_northern_tribes
        requires_dlc_flag = khans_of_the_steppe
        requires_dlc_flag = hybridize_culture
    }