Astra’s hypothesis for why this particular message was previously unsolved is that “TRUPPENVERSCHIEBUNG” was used as the key starting on December 9, 1918 - whereas, as noted above, this message was transmitted earlier, on November 27, 1918. The reason for this discrepancy is unknown.

So people assumed too much, and persistence won the day. Retrying every know password against every undecoded message would have achieved the same result for a tiny fraction of the cost.