Eftersom klonhärden består av kloner av Jango Fett och Jango Fett var Mandalorian och han brukade använda Mandalorian rustning
Wookieepedia säger:
His [Fett's] influence over the clone army's development extended into several areas, such as the resemblance between his Mandalorian armor and the their Phase I armor and the training that molded the clones into elite soldiers.