One can only guess as to the reason. Sometimes things dissapear and reappear from the Discover Cashback pages though, so if you are interested in the gift card or extra cashback, give Discover a call!
I doubt that Discover would want Discover cardholders to apply for someone else's credit products.
Maybe Cabelas didn't see a good enough return for the discount offered thru Discover. Or maybe Cabelas saw a great return, and did sign up tons of folks for their branded card.
Or maybe Discover and Cabelas couldn't agree to terms, such as when cable companies stop carrying certain channels for a while because of contract disputes.
But, to answer your question.... yes, somebody knows!!! It's just not me, is all.