Is a well known fact that the military clothing of Roman high-ranked officers was mostly borrowed from hellenistic examples and remained almost unchangable until Late antiquity. Why? The uniform of Roman soldiers varied greatly from Late Republican times. But why such thing did not happened with uniform of high-ranked roman officers?
In a word, Fashion! Never underestimate its power. The Romans were absolute fashion slaves, and very mindful of tradition and heritage. That was even more important to the upper classes.
Matthew
I'm going to echo Matthew Amt - Yes the Romans were absolute fashion slaves, especially among the upper classes with TRADITION setting the fashion for senior officers. The Romans valued tradition and "the way things have always been done", probably more highly than they should have. Even those officers who started out in a lower social strata, like some during the Third Century crisis, adopted the fashion conventions of the upper classes once they too attained higher status. The right armored appearance, lorica musculata, attic or other "officer's" helmet, conveyed visually one's high social status.