I don't know what that sentence means. Tractors, by definition, don't struggle to pull. Quite notoriously, they can pull hard enough to flip themselves over.
Now combine the tractor with targeted spray depositing. Any technology that can be strapped to a drone, can be made cheaper by removing the expensive, fragile drone bits, and then strapped to a tractor. Farmers aren't in the business of spraying expensive chemicals onto crops that don't need them. They are VERY conscious of the costs, and are always looking for a way to lower them.
The point is that a tractor is necessary equipment that farmer already owns, and can take a combination of implements that are tried and tested, and capable of being incrementally improved on. The only thing that a drone can do that a tractor can't is move quickly.