system
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human
[Lord of the Rings] Why was the balrog still hanging out in Moria?

The balrog was presumably buried deep in the Earth during the last cataclysmic battle with Morgoth, until such time as it was accidentally awakened by the dwarves. After that, and after all the dwarves had been killed, why was it still down there? It was a being comparable in power to Sauron himself, and with no qualms about hiding its power (like Gandalf or Saruman), it should have been ready, willing, and able to emerge into the outer world and begin a rampaging reign of terror across Middle Earth. So why was it still hanging out in some boring old abandoned tunnels, with nothing more interesting to do than scare goblins?
gpt
Balrogs are servants of Morgoth. They were his earliest followers, and they have never demonstrated an inclination to act outside his will. When Morgoth was captured and taken to Valinor, the Balrogs sat in his abandoned fortress and waited. 

You could make the same argument there as you do for Durin's Bane in the Third Age. Outside and to the south were kingdoms of Elves, and a bit to the east of that a few Dwarven kingdoms in the Blue Mountains. Why not rise out of the depths of Angband and scour south, destroying these fledgling upstarts?

Because Morgoth wasn't there to tell them to do that, and that was enough. You might wonder why Sauron made himself a visible player in the Second and Third Ages, and the answer is that Sauron was ever only a servant of convenience. He joined Morgoth because he saw there someone who made plans and followed through, and Sauron wanted to do things. His purpose was always to act. Inaction was what led him to reject the Valar. The Balrogs had no such motivation. They were truly their master's agents. 

So when the Balrog of Moria was awakened, he didn't even kill all the Dwarves. It was a full year before the Dwarves left Moria, and during that time they lost two kings, presumably, leading expeditions into the bowels of their kingdom to deal with whatever terror was picking off their people. When the Dwarves finally flee, no one knows what Durin's Bane was. Apparently, everyone who managed to get enough of a description to help identify the Balrog was killed before they could spread those details. Why did it take so long, and why was it incomplete? Why did the Balrog never venture out, over the course of a thousand years? And why did it not seem to care about the Orcs who arrived five hundred years into its quiet time?

Because it was a servant, and it was waiting for its master, protecting the secret that it was there, still waiting. As it was when the Fellowship came through. The Balrog did not pursue the Fellowship until Gandalf collapsed a room on it.