Every time a shrub is sheared it answers by putting a flush of new shoots out at the cut. Those shoots are all on the outside of the plant, so the outer few inches get denser and denser. That shell then shades everything behind it. After a few seasons of it you have a green skin about two inches thick over an interior of bare twigs, and the plant has no live growth left down inside it.
The problem shows up the first time the shell is broken. A branch pulled out by a storm, scraped by a car door or cut back hard leaves a hole with nothing behind it to fill in, because there is no foliage in there to grow from. A hedge in that condition takes a long time to recover and sometimes does not.
Humidity is the other half of it. A shrub sheared into a solid box holds still, damp air inside it right through the wet season, and a plant with no air moving through it stays wet long after the rain has stopped. That is the condition fungal leaf spots and mildews need to get going.
Pruning is the alternative and it does not take longer. Cuts are made at different depths instead of all at one plane, so the outline of the hedge stays where you want it while light and air still reach the middle of the plant.