Not only was it "simply unavailable" (because "out of print") - anyone attempting to bring in copies of it from outside Germany was subjected to intense scrutiny and required to justify themselves.
Thus, the commercial market was dead. (Because commercial vendors just didn't see the value of bothering.)
Additionally, anyone discovered (e.g., by customs officials) bringing in a privately-owned copy would, of course, been made to "pass through the gauntlet." He would have been required to demonstrate that his copy was a "critically annotated" edition - i.e., one containing abundant footnotes decrying Hitler as a madman.
Hence, it was essentially unprocurable.
Regards,