Similarly, all gates can be realized from only nor -gates. Again an inverter is just a nor -gate with only one input. The main reason is that we want the number of gates of a circuit to reflect the cost of fabrication of that circuit. If we allow arbitrarily complicated gates, we can design an arbitrarily complicated combinatorial circuit with only one gate. Our restriction above is not ideal, since in order for the number of gates to reflect the cost, we would have to take the particular technology used for fabrication into account.
Different circuits are easy to make in different technologies.
But in order to remain somewhat independent of specific technology, we use this approximation.