A small part of it is the aluminum body panels, I think. Aluminum body panels can be repaired, similar to steel, but requires additional training/different tools. The Model S and X are majority Aluminum, whereas the Model 3 is mostly steel with a rear aluminum subframe. Basically if you get in a fender bender you need to go to a specialty aluminum body shop, which don't exist in high quantities outside of south bay.
It also seems like even moderate damage, which would NOT be a write off on an ICE vehicle, results in them totaling it because Tesla refuses to work on anything with even relatively insignificant damage (and they won't touch anything that has a salvage title, so never buy a used Tesla with a salvage title. Ever.).