In answer to the original question it's down to heat and a little bit of aerodynamics. Having ran in crop top and knickers myself I don't overheat nearly as much as running in a vest & shorts. I do like to think that there's less wind restistance wearing less kit too, but I prolly don't run quickly enough to make a difference, but for the elite athletes every little helps, so like the elite swimmers choosing the best costume (even if it only makes a tiny difference) it's still worth the effort.
..and no, the knickers don't 'ride up'
Plus - if you had the body for it wouldn't you want to show it off?