Sheared hedges are made to be dense from top to bottom by shearing them as they develop, from the beginning, when the planting is still new and small. And having the sides angle in so that the bottom of the hedge is wider than the top.

This planting looks like it was bought and installed with the specimens already having been of some size, without having been pruned much beforehand. Also with this particular kind it may be possible there are pests or diseases causing some thinness. If there is much scattered browning of individual foliage sprays or smaller sections this would tend to support that possibility - your pictures are taken from too far away to be able to tell if something like this is a significant factor.

Otherwise if the entire hedge was thin it might be assumed the planting site was too shady for cypresses to grow densely. However the uppermost part is not thin.