Here is the response from Delta (standard lingo):
Thank you for contacting us through delta.com.

As you may know, our schedules load 331 days out, and there is no way to predict when or if Award Seat inventory will be made available.
Fluctuations in passenger demand, seasonal trends, peak travel times and other factors determine if and when Award Seats will be offered, and if so, how many. As a result, the lower level Award Seats may not be available on all flights.

We do value your business and we hope that you will continue to make Delta your airline of choice.