My understanding is that the skateboard would have to be too thick to fit within the shorter wheelbase, substantially raising the occupants, etc. I think this also plays into the chassis design since they say they can build the "chest" design as part of the chassis. I think the skateboard requires more of a traditional body on frame. This is sort of a guess though, but slightly based off of the explanation of how they designed the chassis of the C7 Corvette to be light and more rigid.

Edit: thanks for the additional link