One thing I wonder is whether Nolan failed to even consider how much time & effort the costumes for a film like this would require, both the detail for the main cast & the at least passable costuming for background/extras.
The Prestige was a period piece, but even then, it's unlikely that *everything* needed to be designed & made from scratch. Oppenheimer is also technically a period piece but I am SURE they did not have to fabricate all of the costumes for that.