Why is nitrogen the largest component of the atmosphere?
If it wasn't there wouldn't be any life.
The really is no reason as to why Nitrogen is the most abundant gas in the atmosphere. It is there because it is there. However it also is the only real option for an atmosphere that can sustain life.
However, if Oxygen was the most abundant gas in the atmosphere ...the atmosphere would be too poisonous to breathe, as would many of the other gases that make up the atmosphere (ie. Methane, Ozone, Carbon Monoxide).
The next most abundant gas is Carbon Dioxide. If it was the most abundant gas the greenhouse effect would turn the Earth into a planet similar to Venus, much much too hot to support life. And the same can be said of any of the greenhouse gases (water vapor and methane).