Because it's rare, most likely.
It was banned. So there aren't many copies left, I would think. And also the various Bible Student groups don't re-print it like they do with the other Millenial Dawn/Studies in the Scriptures series.
In my old Hall there was a copy with the missing pages typed up on a eraseable typewriter paper, you know the sticky to the touch kind, and inserted where the banned pages had been razored off.