how rare are power Brakes ?

I'm no expert, wether in brake boosters nor in carburators, but it seems, that the same time, the bigger Thermoquads were used more A-Bodies were equipped with brake boosters.

Could it be possible, that the older Carter and Ball & Ball Carburators could not produce enough vacuum?

No, it was more of the newer, 73 up "A" bodies being other than Hot Rods or econoboxes. They were also a popular rental fleet car in that time period.