Answer:
Because total force applied by horse is greater than the force applied by carriage on horse.
Step-by-step explanation:
Here, the force applied by horse to move the carriage is greater than the force applied by carriage in opposite direction. He exerts greater force with his hooves thus accelerating the cart.
The horse is applying some force to the ground, meanwhile ground is also pushing the horse with same amount of force but as the force crosses the threshold of carriage's resisting force, it started to move.