Probably because they insisted on not using any CGI and doing everything with practical effects instead. I think this is an absolutist take and I would have preferred they used a bit of CGI were it's warranted. Like that nuclear detonation for instance. I couldn't explain why, but compared to the archive footage of nuclear explosion, it just felt wrong.
Flames are chemical reaction. Surface of the flame is where oxygen rich fresh air interfaces with fuel rich, oxygen-depleted hot plasma.
The nuclear bomb can cause secondary chemical fires, but the fireball itself has does not have "flame" look to it.