Here's why:

* Nitrogen gas (N2) is very stable: The strong triple bond between the nitrogen atoms makes it difficult to break apart.

* Most organisms lack the necessary enzymes: To utilize nitrogen, they need to convert it into a usable form, like ammonia (NH3) or nitrate (NO3-). However, most organisms lack the enzymes required for this process, known as nitrogen fixation.

Nitrogen fixation is primarily carried out by certain bacteria, known as nitrogen-fixing bacteria. These bacteria have special enzymes that can break the triple bond in nitrogen gas and convert it into ammonia. This ammonia can then be used by plants and other organisms.

In summary:

* Most organisms can't directly utilize nitrogen gas from the atmosphere.

* Nitrogen-fixing bacteria play a crucial role in making nitrogen available to other organisms.