Why do tractors have large tires?

Tractors must operate on unimproved ground where traction is difficult to come by. The large tires permit the relatively low power of the tractor to pull by avoiding most slippage.
they have large tires so they don't get stuck in mud and soggy grass

Larger tires mean better weight distribution, higher ground clearance and are better for carrying the machine's heavy weight (especially over the machine's lifespan). The trademark grip on a tractor's tire is for soft earth. The weight of the machine, alongside the ballast (liquid inside each tire that adds weight), push the grip into the soil, allowing the machine to pull itself forward.

Farmer's don't usually operate in mud, as most crops should be planted in relatively dry conditions. Although tractors can operate in mud, they have limits. Mud can build up on the tires, covering the grip, decreasing traction. The slippery texture of mud doesn't help with traction either, as does the vehicle's weight, which causes it to sink into the mud. The worst limit is the implement behind the tractor; it can sink in and 'anchor' the tractor in place.
tractors are having broad tyres because larger area will be occupied which will reduce the pressure so ,the tractors can move easily in sandy soil.
Tractors have big rear tires because ,the larger the surface area of the tractor's wheels touching the ground, the more pulling power you have. Like how tractor pullers have large tires. Another reason is that the tractor doesn't have to operate at a higher speed to get the same ground speed with larger tires. Gear ratiOS don't have to be as high b/c of the larger tires.