As Noname remarked, gore would be out of style for TOS. There is also a "zeitgeist" issue. During the '60s Ginger and MaryAnn on GILLIGAN'S ISLAND and Jeannie on I DREAM OF JEANNIE did not show their bellybuttons on TV. The makers of STAR TREK spoke in parables to get their message past the network censors. Add gore to that? Seeing Marta choke in the poisonous atmosphere was enough. No need to Spielberg it up with inyerface FX.
I can see that. I think the problem is that STAR TREK so often had somebody die "to show the situation is serious" (a la GALAXY QUEST), and sometimes many people died, but the show also wanted a lighthearted, happy ending to almost every episode. So that was yet another reason to make the deaths hygienic and tidy.
At the end of "Whom Gods Destroy," if Kirk was having his little joke about Spock letting himself get hit on the head-- but Marta's bloody splash was still imprinted on the viewer's mind, it would clash. If the stained window had been in the actual scene behind Kirk and Spock for that moment, it would clash awesomely and almost sabotage our protagonists. But it's interesting to think about.