Explain why is the distribution of air on Earth like that?

Dear student,
Please find below the answer:

When we study the composition of air, we find that the amount of nitrogen is highest among all the gases. This is because unlike oxygen, nitrogen does not promote burning. It controls fire and if its amount decreases or any other gas like oxygen increases in amount, there will be a disturbance in the ecosystem. There could be uncontrollable fires. In short, any change in the composition of air will reflect badly on the living organisms of Earth.

Hope this clears your doubt.
Regards.