Answer
The deeper you dive, the more pressure is exerted onto your body by the water. After a certain depth, depending on your physique, you will find it impossible to expand the lungs so air can rush in.
Work Step by Step
In order to take a breath, you need to expand your lungs, inside your chest, to create a pressure less than 1 atmosphere, so the air will be pushed through the snorkel down into them.
The deeper you dive, the more pressure is exerted onto your body by the water.
In shallow depths, this may be done, as the water pressure is not that extreme, but as you go deeper, the pressure of the water can make this task impossible.
