Not everyone is included in the SSDI which as has already been pointed out is compiled by the Social Security Admin. from their records and not by Ancestry. No subscription is needed to search the SSDI--although Ancestry pays thousands of dollars to obtain the SSDI and monthly updates, it does not charge researchers to search it. It is provided for free.
For a good explanation of the SSDI and how it works and why someone may not be included in it, see the RootsWeb Guide: http://rwguide.rootsweb.com/
The information can be found in lesson #10.