I'm pretty sure that the cell form factor was a very deliberate cost/benefit decision.
The 18650 cell is the most-produced Li-Ion battery cell in the world, and Tesla has been enjoying the benefits of low prices on these cells as Roadster/Model S/Model X battery pack development has coincided with the switch to laptop form factors where this battery cell doesn't fit. In fact, Panasonic has re-opened production lines for these cells that were shuttered due to falling demand, due to purchase agreements with Tesla. Tesla's incredible demand for batteries means that cell choice is a very important decision. It's worth noting that Tesla's engineering leadership has indicated they are both form-factor and chemistry agnostic, and will always go with the option that makes most economic sense.
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I'm pretty sure that the cell form factor was a very deliberate cost/benefit decision.
The 18650 cell is the most-produced Li-Ion battery cell in the world, and Tesla has been enjoying the benefits of low prices on these cells as Roadster/Model S/Model X battery pack development has coincided with the switch to laptop form factors where this battery cell doesn't fit. In fact, Panasonic has re-opened production lines for these cells that were shuttered due to falling demand, due to purchase agreements with Tesla. Tesla's incredible demand for batteries means that cell choice is a very important decision. It's worth noting that Tesla's engineering leadership has indicated they are both form-factor and chemistry agnostic, and will always go with the option that makes most economic sense.