Prices are falling pretty quick because its a substantial update. You have at least 1.5 stops, Huge DR, and a substantial update to the AF systems.
Its a major update for canon shooters and the price did not go up and in the 1Dx line it went down $500.00 from previous update
old sensors just got very dated.
I think it's this as well. I was expecting that to happen when the 5DIII came out to 5DII prices and it did, but to a much smaller extent and I think it's because the change from the 5DII to 5DIII was not as great as the change from the 5DIII to the 5DIV.