Its just simple math, x amount of dollars for repair versus y amount of value for a vehicle. Replacement parts and shop labor are expensive. Values are low.
The statement about not allowing you to keep it is a little different to me as most places will let you buy back the totalled vehicle, but it may get a salvage title attached to it from then on depending on how they process it, which could be a negative impact if you want to sellor trade it in the future.