Because if you were to study the distribution of stars and other objects like dust clouds, you would find that there are more of these in the Southern Hemisphere of the Milky Way, than in the Northern Hemisphere. Looking up at the sky, the band called the Milky Way seems to divide the sky into two equal hemispheres. But in fact, a series of careful studies at optical, infrared and radio wavelengths shows that these two hemispheres are not exactly equal. Astronomers can use the observations to estimate what our vantage point has to be to create the view that we actually see. The answer seems to be that the solar system is not located exactly in the plane of the Milky Way, but about 40 - 50 parsecs above the plane so that the Northern Hemisphere looks slightly less crowded than the Southern Hemisphere.