Why is there nitrogen in a scuba tank?

It varies. Divers use different breathing gases depending upon their needs.

For standard air, it is 21% oxygen and 79% nitrogen.

For nitrox, it can be any higher mix of oxygen, but typically either 32% or 36% oxygen, and the balance nitrogen.

For trimix, you can have almost any combination of oxygen, nitrogen and helium to suit your depth, and manage your risk to oxygen toxicity and nitrogen narcosis.