The cost of attaching a satellite is more than attaching it to the ISS (which has large solar panels, processors, downlink equipment, etc). We are essentially

The cost of attaching a satellite is more than attaching it to the ISS (which has large solar panels, processors, downlink equipment, etc). We are essentially shipping up an instrument only, rather than the instrument + all the supporting equipment. For a datacenter analogy, it's like adding a HD to a server, rather than buying a new server for more storage. It could run from a sat, but then we'd have much fewer missions we could fly.