Besides the lack of consumer demand (farmers have other things to worry about), I think the other thing that makes the farm market slow to adopt electric is that the market is, erm, slow. People are constantly buying new cars. Some people drive a new car every year, some people drive a car for 10 or 20 years, but I'd guess that most people get a new car every 3-7 years. That means, firstly, that a lot of people have bought a new car since they first started getting good; that also means that an electric car only has to last 3-7 years to meet most people's expectations, and your car will only be 3-7 years out of date when you get a new one.
I would guess the average farm tractor in the fields today in the US is 20-30 years old. You've got some really new ones with GPS, autopilot and satellite TV, sure, but you've also got some 50-75 year old tractors which are still doing just fine. I used to rake hay with a little Farm-All my great grandfather bought back in his prime, which is still going strong today, and I bought a used Cletrac from '45 to use around my woodlot.
This creates three problems for the prospective electric tractor manufactures: not a lot of tractors are sold each year, so you're fighting for a chunk of a very slow-moving market; prospective buyers are going to be asking themselves "will this tractor outlive me?" so you need to have a compelling story on how those battery packs and motor bearings are going to last 50 years, or be easily replaceable; the technology is still rapidly developing, so you also need to explain to the prospective buyer why it's is worth buying an electric tractor now, instead of waiting 10 years to buy a more mature electric tractor.
It's actually surprisingly hard to find hard numbers on this topic, but the numbers I've seen have generally suggested that the average age is more like 10-20 years old. The hard numbers I've seen do say that about 10-20% of tractors are less than 5 years old.
Realistically, the difficult part of electric tractors isn't the mileage but rather the fact that you're consuming far more power per mile. You're looking at measuring fuel consumption of around 10 gallons an hour, instead of about 1.5 gallons an hour for a car. Also, you're going to want to work the tractor for as much sunlight as you can muster, and you want to charge back up only overnight. So you'd want an equivalent not of a 200 mile range car but more like a 2500 mile range car, that can fully charge overnight.
Re: energy storage, oof, yeah, at 146.5 MJ / gallon for diesel fuel, and 10 gallons / hour, 12 hours of fuel holds 4884 kWh of energy. Assuming you need a similarly sized battery, you're looking at 60 Tesla (car) batteries. That will weigh approximately 36 tons and cost about $700,000, based on Tesla's 141$/kWh stated cost.
Hmmm, I'm guessing you can't put more than 8-10 tons of battery on a tractor that size (the John Deere 7250, which is actually 13g/hr, weighs 13 tons) so you're limited to maybe 3 hours or so of power between charges or swapping out your batteries. For that much battery, you're looking at maybe $175,000; the John Deere 7250 retailed for ~$230,000. Assuming your batteries are swappable and fast-chargeable, you'd need at least two batteries. (Heck, it's a tractor, maybe it can be on some sort of wagon you tow behind, which would make swapping a snap.)
Putting it together, I think we might actually be approaching viability for electric tractors. If you can actually fit 8-10 tons of battery into a 13 ton tractor and build the rest of the tractor for $55,000, you could build a tractor comparable to the John Deere 7250 that would run for about 3 hours; that's not a bad amount of time to run between swapping out the batteries, but you'd need at least another $175,000 battery to swap out, which already makes you ~twice as expensive as a regular tractor. You would probably need the battery weight and cost to drop by half again before you could get a tractor that would run all day (with a battery swap) and not cost more than a conventional tractor.
Well, if you're towing something behind you on a tractor, you can't use the power take-off anymore, which kind of ruins the point of the tractor for most uses.
You might be able to set up a convoluted system where you have the power cells being dragged behind whatever equipment you're using, assuming you can get enough tires underneath to not undo whatever work you're doing to begin with (first we plow the field, then we pack it down tight!)
But then you also can't use the ~10 tons to provide extra weight on your wheels.
It just feels like you ought to be able to store your batteries in a way which makes swapping them/carting them around easier, if you don't need to have a sleek aerodynamic package that fits on a highway. Actually requiring a crane to swap your batteries adds a lot of overhead, along with room for comic fuckups.