One of the reasons is that, with binoculars, an observer can use both of his or her eyes. Our perceptions of color, contrast, and resolution improve significantly when using two eyes instead of one (like when using a telescope).
Try this test on a clear, moonless night:
With both eyes open, cover one with your hand and look at the sky. Pick out a specific area and make a mental note of the faintest stars that are visible. Now uncover your second eye and look at the same area.
When you do this, you should find that you can see even more fainter stars, gaining perhaps a 10 percent improvement in your perception. Harrington adds that the increase is even more noticeable — up to 40 percent better — when looking at nebulous objects, such as the Milky Way.