I believe cabin furnishing and ground testing is usually done in the same station at the same time. I'd imagine airlines choosing to not get a line fit do so as part of the order and it's possible Airbus isn't equipped or doesn't want a frame to go through the same station twice and take up extra time/resources. At this point, it's likely just better for everyone to delay a bit instead of Airbus delivering without interior and JAL finding a place that can install the interior on short notice while managing the logistics of getting all the parts from TLS to whoever is going to do the install.