The tractor has a 100kW drive motor and 200kW for powering implements and they run 2.5kV in the cable. To put in perspective as to how large your service drop has to be, 240V requires 1250A for 300kW. One needs 1500A (only use 80% for 100% duty cycle) service in the field just to power one tractor.
At 2500V they only need 120A in the wire strewn across the field. A flexible 1000m cable for 120A at 100% duty cycle is going to be at least $40,000. And how often to replace or repair from dragging it around?