The original motive was that enough batteries for a truck were expensive, and if you did a radical redesign with a folded exoskeleton and didn't paint it you could radically reduce weight and cost, making an EV truck possible. They were also developing a next generation battery to make this feasible.
A lot of time time passed. 3rd party batteries got cheaper/better. Tesla abandoned the exoskeleton idea and failed to hit its battery targets.
So now they're putting a lot of effort into building a funny looking truck that is all form and no function while traditional truck makers are just jamming a lot of batteries into a standard truck and achieving something better on many dimensions.