Answer:
It is because of the tilt of the earth.
Step-by-step explanation:
the earth is tilted at 23.5 degrees. this makes it so that either the northern or southern hemisphere will be exposed to more rays from the sun. In the areas that are getting more rays from the sun, it gets warmer. Think about it like this, because the earth is tilted, part of it is more in the shade and part of it is more in the light. And its colder in the shade, so thats why seasons happen and why they dont happen at the same time.