Without going into too much detail, basically the axon is still has some potassium permeability even with the myelin sheath (although it is lower). Leakage of potassium out of the axon causes repolarization of the axon cytoplasm. This occurs even when you have a myelin sheath. So as the action potential is traveling down the axon, it is getting less and less positive. At some point, if you don't have another sodium channel to re-depolarize the axon cytoplasm, enough potassium will leak out and the axon cytoplasm will return to its basal electric potential. So these nodes of Ranvier are spaced out so that when the action potential reaches the next one, the potential will still be above the threshold for depolarization, and it will cause sodium to enter the axon and further propagation of the action potential.
