Just wondering what possible reasons for seat selection being unavailable are. Was about to book a SYD-AKL flight for early November, but seat selection is unavailable for that flight, and the most common reason I'm aware of for that is the hardware is changing.

Flight is currently listed as a 789, and if its going to be subbed out I'll grab an earlier flight, since the only reason I selected that flight was to tick the Dreamliner off the list.

What other reasons are there for seat selection being unavailable?