Why were the Triarii differently equipped?

[Youngster]

Well, there are more and less solid versions. Some of them are even fully closed, with the eye hole merely incised patterns. In such cases they were probably fairly protective. But I don't know as to whether those were the versions most favored by the Romans.
I think it just comes down to a matter of fashion. The fact that the AP helmet recalled the the pushed-back corinthians shown on red figure pottery probably made the soldiers wearing them feel heroic. And the Romans, throughout their history, always seemed willing to sacrifice a bit of practicality and cost effectiveness if it looked good. Tall crests look good, but they are handholds for enemy soldiers. The same can be said of the balteus of the principate. Likewise, there was no need to have richly decorated scabbards; but they did. I think that we often forget, as modern folk inclined to scientific thinking, that the ancients had sometimes "irrational" values. Maybe they knew the helmets were less effective, but thought that the intimidation benefit outweighed the weakness. Maybe the protective difference was never really discovered because the helmets were not subjected to a controlled test (though I think they had enough experience to figure this out). Maybe the purchase of AP helmet was just something senior soldiers did, and they never questioned why.