I guess it always seemed like when the Enterprise left, teams of experts would head to planets to pick up where the mission left off. Kirk's crew were the paramedics/ER docs - responding to the environment/situation they found themselves in - and the experts who came after were the surgeons/ongoing care specialists/research lab scientists - handling the long term issues and doing ongoing study. Not to say that Kirk's crew couldn't continue to be involved/study/write papers on what they uncovered, but the episodic nature of TOS didn't usually suggest/depict that they were - always off to a new adventure, which became the concern of the moment.