As stated in the Terms & Conditions on the page:
Eligibility:
Only members who receive this communication directly from Delta Air Lines are eligible to participate in this offer. This offer is non-transferable. To participate in this offer, eligible members must register online by January 24, 2010.
It is a targeted promotion, and figuring out how DL determines who is targeted is an exercise in futility.
Note: I tried to register for the promotion, and I was not eligible.