In mutual freefall, on orbit, it's possible to take as much time as you want, reducing relative velocities as much as you want, and keep things gentle.
When landing on the Earth (or a barge) you can only control speed so much before you run out of fuel, so you're stuck with high speeds, high accelerations, and large forces. The margins are much thinner and you have an extremely limited amount of time to get things done.
Additionally, the landings are an R&D program being carried out for very little cost, the ISS cargo trips are a commercial service being bought at around $130 million per flight, so the engineering resources available to either are considerably different. If SpaceX could plow hundreds of millions of dollars just into developing the operational capability of landing their rockets they'd likely have done it many times by now.
In mutual freefall, on orbit, it's possible to take as much time as you want, reducing relative velocities as much as you want, and keep things gentle.
When landing on the Earth (or a barge) you can only control speed so much before you run out of fuel, so you're stuck with high speeds, high accelerations, and large forces. The margins are much thinner and you have an extremely limited amount of time to get things done.
Additionally, the landings are an R&D program being carried out for very little cost, the ISS cargo trips are a commercial service being bought at around $130 million per flight, so the engineering resources available to either are considerably different. If SpaceX could plow hundreds of millions of dollars just into developing the operational capability of landing their rockets they'd likely have done it many times by now.