Your question would be a good one to post to the Washington Post's weekly chat on travel.

I think what has happened is that the bulkhead seats you need for the bassinet have been taken by travelers who have status within Delta's skypesos program -- passengers who are silver or higher within the skypesos program can pick those seats, while passengers without skypesos status cannot.

Bulkhead row seats seem to be pretty popular seats, judging by how many past FT threads involve comments about other passengers resting their bare feet on the bulkheads.