for me it is personal preference but I also ride different crank lengths on most of my bikes (different bikes different purposes) i.e. my touring bike with wide gears is 171mm
my fixed gear conversion is 171mm also - made the transiion easy
my track iron is 170mm
and my other track bike_mostly for street use is 165mm (this is helping me to spin more evenly)
on my dirt jumper 180mm
and like others have said it also has some to do with your body structure