Well as F76 will be a prequel they could explain a new type of PA as some sort of a "only a few units (like 10 or so) werte produced as very secret prototypes" that were only tested in that very area, maybe even as specialized armor for securing non experimental vaultslike 76 or something like that.
But then they would have to make sure there won't spawning more than those 10 sets of armor, so there is no chance it will appear hundreds of years later in DC, Vegas or whatever...
The PA from the E3 trailer seems to be some kind of construction site or mining armor, so maybe it's original functionality was limited and therefor never got out of mining areas...