To be clear, the seats the OP is seeking on the 777 are not EC. I was just giving an example of some other seats that I know to always be blocked until T-24, which do happen to be EC -- the seats I mentioned on the E-Jets and CRJs cannot be assigned (AFAIK) until T-24, even if you are entitled to or willing to pay for EC.

In the case of the seats on the 777, obviously they are desirable, for the exact reasons the OP wants them, so they may be held back for elite customers or something like that, but I would think if so, Delta would just make them "Preferred" seats rather than blocking them. Rather there just seems to be a blanket rule that the last two rows of every cabin is blocked until some time closer to departure, perhaps to give Delta some flexibility to get families seats next to each other if they weren't able to get them when booking.