Weight and cost. But most trucks probably don't run near 80,000 lbs. I'm going to guess that a typical Walmart semi delivery is 40-50k lbs. If that is true, Walmart would be interested only in cost per mile. They could run a 1 mWh of batteries in a semi if that made financial sense.
The prototype looks like it can get 400-600kWh behind the driver, using the power pack as a volumetric guide. Tesla can do a longer wheelbase if needed to add capacity.
The volume of a sleeper cab type semi is considerably greater than most people realize. Probably more space than Tesla can afford to fill with battery.
Funny you should mention Walmart delivery trucks. A friend of mine does exactly that for Walmart and for some reason the weight did come up when he was here this summer. I believe he said the trucks were typically 40,000 pounds, though I don't recall if he meant total or just payload. The type of stuff those store delivery trucks haul is lower density than something like lumber or rebar.
The advantage with a store delivery truck is they stop at several stores in a day. It takes some time to unload the cargo for that store and the driver usually has a union mandated break. It would be easy to put a fast charger on the loading dock and give the truck a boost while it's unloading. Some stores are putting solar on the roof now, so that would make it even cheaper to run electric trucks.
The store might not want to dedicate the power for a charger that runs 350 KW, but they could put in one or two circuits capable of 150 KW which would give the truck another 100 miles or so in an hour of sitting idle. With that model, a semi could run the normal route an ICE would run with no more down time than the ICE would have in the natural course of events.
That may be Tesla's initial target market for the initial semi, store delivery trucks. Elon said they already had one operator with a large fleet signed on and that could be Walmart. Walmart is always looking for ways to cut costs and this would cut costs.
I have been scratching my head for months trying to figure out why Tesla was aiming for the over the rig market first when there was plenty of low hanging fruit in the short haul market and Elon never goes out on a limb unless he's 99.999% sure it will hold up to the strain. And he's very thorough.
I was thinking of the long haul trucking market like the coast to coast haulers. A standard class 8 semi has around 600 miles of range in a base configuration, though extra fuel tanks can be added. Having to shut down on the road for 1-2 hours to supercharge after 300 miles of driving would be unacceptable to a lot of operators who run a straight 600 miles without stopping (or only brief stops) now.
Fatigue rules do require drivers to take breaks, but the truck can keep moving if there are two drivers. My friend who works for Walmart was on their moving crew for a while. They had a couple of truck drivers who would move managers from one store to another and they always had a pair of drivers who would swap off to keep the truck moving all day.
In any case, if they aren't going to the real long haul trucks and targeting store delivery trucks running out of distribution warehouses and chargers could be installed at stores to give the trucks a bump while unloading, that would be very doable. I could see a lot of stores going for that. It would save them a fortune.
