The concentration of sodium ions is greater outside the axon than inside the axon during the resting potential. why is that the case?

Because through this the membrane potential is maintained around -70 mV inside the axon.

Explanation:

The transmitting of nervous impulses is depending on an electrochemical mechanism. This mechanism uses ions for inducing electric charges between both sides of the membrane. Sodium and potassium are the most important ions during this process, because depending on the concentration of them in and outside of the membrane, the electric charge is determinated.

During the resting potential there is a higher concentration of sodium ions outside of the axon because through this the membrane potential is maintained around -70 mV inside the axon. Then, when the neuron is going to transmit the impulse the sodium and channels are opened for allowing the entry of sodium ions and to elevate the membrane potential until 40 mV.