Salt water isn't good for rockets. Once you solve this really hard problem of landing a rocket on a barge on a dynamic ocean you have a real competitive advantage.
Is that the reason they've been using the barges for landing? I'd always assumed they were just being extra cautious since it could endanger civilians or crew if the reusable stage was supposed to land on solid earth.
I'd imagine that the barge landings introduce a lot of uncertainty and complicating factors that are impossible to completely nullify. High waves and (comparatively) strong open-ocean winds seem (to me at least) to make consistently successful lands much harder.
There was definitely a safety issue that lead to earlier landings taking place on a barge, but it also requires a lot less fuel to land somewhere downrange (which is always over the ocean) rather than boost all the way back to the launch site.
Some flights wouldn't have enough fuel to make it back to the launch site. That's the reason I've seen. There is also a large amount of water, so that increases the places you can land.
I'd imagine that the barge landings introduce a lot of uncertainty and complicating factors that are impossible to completely nullify. High waves and (comparatively) strong open-ocean winds seem (to me at least) to make consistently successful lands much harder.