IANARS (I am not a rocket scientist). My guess is that it primarily an issue of time. When you are trying to dock with the ISS, you are effectivly in a 0g envi

IANARS (I am not a rocket scientist). My guess is that it primarily an issue of time. When you are trying to dock with the ISS, you are effectivly in a 0g environment, so you can take as much time as you want. This means that you can cancel out your relative velocity at a safe distance and crawl your way towards the ISS at whatever speed you like. When you try to land on a barge, you essentially need to do a suicide burn, where you wait until the last second to fire your engines enough so that you have zero relative velocity at the instant you hit the barge [0]. Added to this is the fact that landing is largly an afterthought.They would not build, let alone launch, the rocket before they were confident it could dock. However, because the marginal cost of a landing attempt is (relativly) low, they decided that it is cheaper to attempt landings with low confidence and get data instead of attempt to engineer a perfect landing with high confidence before they attempt anything. [0] If I recal correctly, their margin of error is stopping within one second of hitting the barge.