Why was a thin layer of onion cells used?

A light microscope functions by shining a light from below, through the specimen on the slide, and then through the objective and eyepiece lenses to the observer's eye. For an image to be formed, light must be able to pass through the specimen. If the layer of cells is too thick, it will be opaque, blocking the light. Therefore, a very thin layer, ideally only a single cell thick, is prepared so that it is transparent enough for light to pass through and reveal the internal structures of the cells.