It's because of the way the Earth turns. When the Earth is viewed from space, cloud patters form great swirls (known as cyclonic patterning) across the continents. In the Northern hemispher they turn anti-clockwise, in the Southern Hemispher clockwise as the Earth turns from left to right, dragging the atmosphere and its cloud formations in line with the equator. In the British Isles, we get prevailing winds blowing eastwards off the Atlantic.