Or higher voltages. You really need to be able to transfer a lot of power for at least 1000 to 2000 km with at least a half decent efficiency.
The calm periods are in large high pressure areas, thus the power must be injected into that area from some windy areas around it.
Look at a meteorological chart and especially at the isobar (equal pressure)curves. The more dense these curves are around a center of low pressure, the higher the winds and hence available power.
In a large high pressure area, there are very few (or none) isobar curves, thus no wind and hence no wind power.