Well, it was threatened by proxy by Lazarus and the Giant Space Amobea, who threatened the universe or the galaxy. One could also argue that Khan was a threat for Earth, if he had defeated Kirk, but then again you could make this claim about most baddies with ambitions.
Nomad and the Doomsday Machine threatened Earth, but they were dealt with long before reaching it.
The fact remains that the drama of the week mostly - often entirely - based upon how the main cast would work its way out of the current crisis. Hell, for the most part TNG was as well. This is a smart thing to do, because all good drama with emotional resonance is based upon individual characters. For a real world example, look at how the refugee crisis in Europe was mostly ignored by the U.S., until one drowned boy washed up on the beach and was photographed. Or how war movies almost always focus not on the "general's room" but on individual soldiers fighting for their lives. People just can't relate emotionally to the epic scale of a war - even when they do understand it intellectually. They can relate to the trials and tribulations of a small group of well-defined characters.
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