Article in todays WSJ says that Tesla is having a big problem with the choice of ultra-hard stainless steel.
"...
The stainless steel, which is being used for the truck’s outer shell, has proven challenging to bend and manipulate, testing Tesla’s ability to shape the material into body panels that line up correctly and don’t result in large gaps when installed, according to people who have worked on the pickup.
On top of that, it is so hard and strong that it can be difficult to flatten, these people said. The metal is produced in coils that resemble giant rolls of toilet paper, and even when unrolled, it has a tendency to spring back into its earlier, curved form. ...:"
Basically it's too hard to manipulate using typical stamping machines.
There's a reason only one other production car used a stainless steel body, and it was a failure.