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The short answer is that Middle Earth has been tainted and permeated by the evil of Morgoth, such that everything is involved.
To gain domination over Arda, Morgoth had let most of his being pass into the physical constituents of the Earth – hence all things that were born on Earth and live on and by it, beasts or plants or incarnate spirits, were liable to be ‘stained’.
(From Morgoth's Ring - volume XI of the History of Middle Earth)
To eradicate evil would, therefore, entail eradicating everything - including all the elves, men, dwarves and ents; all the land itself.
Yet destruction of Middle Earth and all in it would itself be a great evil.
The situation seems insoluble...
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And this is Tolkien's fictional and sub-created 'explanation' of the necessity for the incarnation of Christ, His death and resurrection; because it is necessary for everything - humans and the earth itself - first to die, then to be remade perfect; and this is the only possible cure for evil.
Paradise, complete Good-ness, necessarily lies on the other side of death and destruction - as described in the Revelation of St John (upon which Tolkien had meditated deeply while translating Pearl by the Gawain poet).
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