Because the Earth's axis is not "straight up and down" as we move around the sun.
So when we're on one side of the sun, the top pole leans slightly toward the sun. During that time the sun shines more directly on the top half of the Earth, and less directly on the bottom half. The people on the top half see the sun higher in the sky, and their weather is warmer, while the people on the bottom half see the sun lower in the sky, and their weather is cooler.
Then, when we're on the other side of the sun, the top pole leans slightly away from the sun. During that time the sun shines more directly on the bottom half
of the Earth, and less directly on the top half. The people on the bottom half see the sun higher in the sky, and their weather is warmer,
while the people on the top half see the sun lower in the sky, and their weather is cooler.
The Earth makes the complete trip around the sun in one year, so the people on the Earth go through this cycle of higher/lower sun and warmer/cooler weather every year.
