While I personally think that anyone buying a "pickup truck" with a bed shorter than six feet is a fairy, my opinion is obviously in the minority. Even the most humoungous trucks these days are four-door vehicles with pathetic five foot (60 inch) beds. This trend appllies to both full-sized and mid-size trucks.
What appears to matter to pickup buyers is PAYLOAD and TOWING CAPACITY, by which measures the Tesla "doorstop" measures up nicely. Indeed, if you are willing to pay extra for more juice, you can get both payload and towing comparable to the best standard (half-ton) trucks, albeit with dramatically reduced range under load.'
For those who want the traditional eight foot bed, Tesla has no answer. So they walk away from 20% of the customer pool.