Por que Sauron não guardou o Monte Doom?

É bem simples: Sauron não esperava, e não podia conceber, alguém realmente tentaria destruir o Anel em vez de reivindicá-lo por si mesmo.

"He is in great fear, not knowing what mighty one may suddenly appear, wielding the Ring, and assailing him with war, seeking to cast him down and take his place.That we should wish to cast him down and have no one in his place is not a thought that occurs to his mind. That we should try to destroy the Ring itself has not yet entered into his darkest dream."

-The Two Towers, "The White Rider"

Essa completa falta de desejo de qualquer tipo para dominar o Anel foi a principal razão pela qual Frodo foi escolhido para ser o Portador do Anel. Até mesmo Gandalf recusou-se a tocar o Anel por medo de ser consumido por ele e não ser capaz de destruí-lo.

Esse foi um ponto-chave da estratégia de Gandalf e Aragorn, e todo o motivo eles lideraram o exército de Minas Tirith até o Portão Negro de Mordor. A esperança era atrair não apenas a atenção de Sauron, mas também seus exércitos, deixando Mordor em si desprotegido.

Livro 5, cap. 9:

[Gandalf]: ‘His doubt will be grow­ing, even as we speak here. His Eye is now strain­ing to­wards us, blind al­most to all else that is mov­ing. So we must keep it. Therein lies all our hope. This, then, is my coun­sel. We have not the Ring. In wis­dom or great folly it has been sent away to be de­stroyed, lest it de­stroy us. With­out it we can­not by force de­feat his force. But we must at all costs keep his Eye from his true peril. We can­not achieve vic­tory by arms, but by arms we can give the Ring-bearer his only chance, frail though it be.

Além de todos os outros comentários, é difícil vigiar o Mount Doom. Ele entra em erupção regularmente, então ter soldados em guarda próximo a ele presumivelmente eleva o risco muito real de eles serem queimados em cinzas. A estrada para o Monte Doom tinha que ser regularmente limpa e mantida com grande esforço de acordo com os livros, então o grupo mais provável de servos Sauron que você provavelmente encontraria lá provavelmente seria uma equipe de reparos de estradas ...

O próprio Mordor era realmente muito vigiado, com legiões de tropas orcs ocupando-o, como aprendemos na Terra da Sombra :

Frodo and Sam gazed out in mingled loathing and wonder on this hateful land. Between them and the smoking mountain, and about it north and south, all seemed ruinous and dead, a desert burned and choked. They wondered how the Lord of this realm maintained and fed his slaves and his armies. Yet armies he had. As far as their eyes could reach, along the skirts of the Morgai and away southward, there were camps, some of tents, some ordered like small towns.

O fato é que Frodo e Sam levaram uma porta dos fundos pouco conhecida para Mordor; a entrada principal (via o Portão Negro) foi efetivamente intransitável (de O Portão Negro está Fechado ):

Across the mouth of the pass, from cliff to cliff, the Dark Lord had built a rampart of stone. In it there was a single gate of iron, and upon its battlement sentinels paced unceasingly. Beneath the hills on either side the rock was bored into a hundred caves and maggot-holes: there a host of orcs lurked, ready at a signal to issue forth like black ants going to war. None could pass the Teeth of Mordor and not feel their bite, unless they were summoned by Sauron, or knew the secret passwords that would open the Morannon, the black gate of his land.

E até mesmo a entrada dos fundos que eles levaram também foi bem guardada ( A Torre de Cirith Ungol ):

Since his return to Mordor, Sauron had found it useful; for he had few servants but many slaves of fear, and still its chief purpose as of old was to prevent escape from Mordor. Though if an enemy were so rash as to try to enter that land secretly, then it was also a last unsleeping guard against any that might pass the vigilance of Morgul and of Shelob.

Para realmente fazer qualquer coisa útil no Mount Doom, você obviamente tem que entrar no Mordor primeiro, o que é algo que não se faz simplesmente.