Simple: they were following prevailing winds and currents. If you follow modern ocean sailing races, they follow the same routes. Note that as boats approaches the cape of good hope from the east, they often zig zag, fighting the oncoming winds and currents around the cape. Once they round the cape, its a straight shot for the north atlantic. Another interesting route: from the east coast of india to the west - they had to loop way down south in the indian ocean to find favorable winds. Going the other way, its a straight shot.
In the north atlantic, wind and currents generally follow a clockwise rotation. In the south atlantic, generally counter-clockwise. Same for the pacific.
