Logically explaining the earth's magnetic field is aligned in the north direction, thus directing magnetic energy and the sun rises in the east, giving solar energy. And this is the reason why north, east and north-east directions are considered auspicious / fruitful directions. Now, speaking about south, west and south-west directions, these are not the major magnetic or solar energy directing points and as the day proceeds the sharp rays of sun tend to align in these directions (since sun sets in west). Not just the sharp rays, but also harmful UV rays might enter in this direction. Thus, it is preferable not to have the entrance at the south-west direction.