Por que os Valar abandonaram os Moriquendi e os Homens?

Tolkien aborda isso um pouco em um ensaio intitulado "Notas sobre motivos" (grifo meu):

Morgoth lost (or exchanged, or transmuted) the greater part of his original 'angelic' powers, of mind and spirit, while gaining a terrible grip upon the physical world. For this reason he had to be fought, mainly by physical force, and enormous material ruin was a probable consequence of any direct combat with him, victorious or otherwise. This is the chief explanation of the constant reluctance of the Valar to come into open battle against Morgoth. Manwë's task and problem was much more difficult than Gandalf's. Sauron's, relatively smaller, power was concentrated; Morgoth's vast power was disseminated. The whole of 'Middle-earth' was Morgoth's Ring, though temporarily his attention was mainly upon the North-west. Unless swiftly successful, War against him might well end in reducing all Middle-earth to chaos, possibly even all Arda. It is easy to say: 'It was the task and function of the Elder King to govern Arda and make it possible for the Children of Eru to live in it unmolested.' But the dilemma of the Valar was this: Arda could only be liberated by a physical battle; but a probable result of such a battle was the irretrievable ruin of Arda.

^{History of Middle-earth X Morgoth's Ring Part 5: "Myths Transformed" Chapter VII "Notes on motives in the Silmarillion" (ii)}

A disseminação de seu poder por Morgoth na questão de Arda (que ele vinha fazendo basicamente desde os anos das lâmpadas; esse não era um problema novo na Primeira Era) criou um cenário invencível para os Valar. Ou eles destruíram Morgoth, destruindo a maior parte de Arda no processo, ou deixaram que ele fosse e ele faça ele mesmo.

Tolkien sugere ainda que, do ponto de vista dos Valar, os Noldor estavam prestando um serviço muito útil, mantendo Morgoth bastante ocupado e contido, e também elevando a raça dos Homens:

If we consider the situation after the escape of Morgoth and the reestablishment of his abode in Middle-earth, we shall see that the heroic Noldor were the best possible weapon with which to keep Morgoth at bay, virtually besieged, and at any rate fully occupied, on the northern fringe of Middle-earth, without provoking him to a frenzy of nihilistic destruction. And in the meanwhile, Men, or the best elements in Mankind, shaking off his shadow, came into contact with a people who had actually seen and experienced the Blessed Realm.

In their association with the warring Eldar Men were raised to their fullest achievable stature, and by the two marriages the transference to them, or infusion into Mankind, of the noblest Elf-strain was accomplished, in readiness for the still distant, but inevitably approaching, days when the Elves would 'fade'.

^{History of Middle-earth X Morgoth's Ring Part 5: "Myths Transformed" Chapter VII "Notes on motives in the Silmarillion" (iii)}

Mais tarde no ensaio, Tolkien observa que a intervenção dos Valar na Guerra da Ira foi na verdade um movimento calculado; Morgoth foi severamente enfraquecido e contido em uma única região de Arda. Assim, ele poderia ser derrotado mais facilmente, e apenas arruinar parte do mundo, em vez de tudo:

The last intervention with physical force by the Valar, ending in the breaking of Thangorodrim, may then be viewed as not in fact reluctant or even unduly delayed, but timed with precision. The intervention came before the annihilation of the Eldar and the Edain. Morgoth though locally triumphant had neglected most of Middle-earth during the war; and by it he had in fact been weakened: in power and prestige (he had lost and failed to recover one of the Silmarils), and above all in mind. He had become absorbed in 'kingship', and though a tyrant of ogre-size and monstrous power, this was a vast fall even from his former wickedness of hate, and his terrible nihilism. He had fallen to like being a tyrant-king with conquered slaves, and vast obedient armies.

The war was successful, and ruin was limited to the small (if beautiful) region of Beleriand.

^{History of Middle-earth X Morgoth's Ring Part 5: "Myths Transformed" Chapter VII "Notes on motives in the Silmarillion" (iii)}

Eles não queriam mais se intrometer nos assuntos da Terra-média. Convidar os elfos para Valinor, embora tenha sido feito com boas intenções, acabou resultando na destruição das Duas Árvores e no "escurecimento de Valinor". Eu acho que os Valar tiveram uma grande surpresa depois disso.

Há outra fonte de ajuda que os Valar forneceram à Terra-média que as pessoas deixaram de mencionar. Depois que as duas árvores morrem, os Valar pegam os dois últimos frutos da árvore (o sol e a lua) e os colocam em órbita ao redor do mundo.

Essa ação basicamente acabou com os planos de Morgoth de dominar a Terra-média. Morgoth e seus servos retorcidos não podiam tolerar o sol e limitavam-se a esconder-se no subsolo ou em torno de suas vastas masmorras vulcânicas, onde a poeira vulcânica bloqueava o sol.
O próprio Morgoth odiava o sol e não o suportava, nem os orcs. Essencialmente, esse ato colocou uma enorme desvantagem em seus planos de dominar a Terra. Ele finalmente conquistou Beleriand, mas foi necessária a ajuda dos dragões e homens maus para conseguir isso.
No entanto, foi uma vitória vazia para Morgoth, que agora estava essencialmente preso no subsolo - dificilmente uma estatura impressionante para o "senhor de Arda"