Porque o exército de clones consiste em clones de Jango Fett, e Jango Fett era mandaloriano e ele estava acostumado a usar a armadura Mandaloriana
Wookieepedia diz:
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.