Because there are already regular delivery trips to the ISS, and the cost of such experiments is thus substantially subsidized by ISS operations. A cube sat wou

Because there are already regular delivery trips to the ISS, and the cost of such experiments is thus substantially subsidized by ISS operations. A cube sat would have a very limited lifespan and cost more than their program budget on its own, besides being too limited to be useful. If their program proves successful they might be able to raise enough money to put hosted payloads on other spacecraft, one of the best being Iridium NEXT. But for now the ISS is their best shot.