Why is nitrogen made up of so much of the air?

Question

Why is nitrogen made up of so much of the air?

Answer 1

mostly because it is basically inert and therefore remains in the atmosphere

Answer 2

You mean, "Why is the air made up of so much nitrogen?"

It wasn't always that way.  Way back in the Earth's history the atmosphere contained a lot more water vapor, CO2, ammonia and lesser amounts of other gases, like methane.  Ammonia was produced by the many more volcanoes.

The nitrogen that we have today came primarily from the decomposition of the ammonia caused by the UV light from the sun.  The hydrogen that was also produced, being much lighter, was more prone to move to the top of the atmosphere and be lost into space.