Some of the Redditors have their Lumia 1520 shipment delayed due to “overwhelming demand” of the device as mentioned in the email received by them. Now, it may be possible that AT&T which has been infamous for always having ridiculously small stocks of earlier Lumias as well, has run out of its first stock.
Here is the Reddit discussion link anyways.
Now, in case you are worried about how to get the device early in face of these shipping delays and haven’t ordered one yet, then you can order it at Amazon and get the device by Nov 26. Black Lumia 1520 is in stock at Amazon and priced similar to AT&T starting at $99 for 2-year contract plans. Other colors are not yet in stock at Amazon though.