Though the Valar delivered the Elves from Morgoth and made war on him for this purpose, it seems to me that they were less kind to the Secondborn.
Consider this. When the Vala Oromë found the first Elves dwelling beside Cuiviénen, he sped back to Valinor with great joy. After informing the other Valar that the long awaited Children of Illúvitar had at last awoken, he swiftly returned to Middle-Earth to dwell among his beloved people. Okay, there's nothing wrong with that. He liked Elves, that's all.
And although they were less fanatic about them than Oromë, the Valar liked Elves too. They took counsel; and to deliver them from the dominion of Melkor they made war upon him and took all of the Eldar that were willing back to Valinor. That was a noble act, and the Valar made a good choice in performing it. But for them the Elves would be only a shadow of what they became.
Many years later, the first fathers of Men awoke somewhere in the South of Middle-Earth. At first they were few and weak, and Morgoth corrupted them with relative ease when he got the chance. However this time, although the Lords of the West knew of their existence, there was no heroic salvation of the Atani from the Shadow.
Was this because Melkor was less of a threat to Men than he had been to Elves? In the beginning that was certainly true. The strength of Morgoth was diminished for a time, with the onset of the light of the Sun. But after his armies had got used to this light, he arose to his former might once more. Even then, no rescue attempts were made.
Men are different from Elves; of course, there's no doubt about that. Even if they had been ferried back to Valinor the two Kindreds would never become like each other. So Illúvitar in the Silmarillion states:
" 'Behold I love the Earth, which shall be as a mansion to the Quendi and the Atani! But the Quendi shall be the fairest of all living creatures, and they shall conceive and bring forth more beauty than all my Children; and they shall have the greater bliss in this world. But to the Atani I will give a new gift.' Therefore he willed that the hearts of Men should seek beyond the world and find no rest therein; but they should have a virtue to shape their life, amid the powers and chances of the world, beyond the Music of the Ainur, which is as fate to all things else. And of their operation everything should be, in form and deed, completed, and the world fulfilled unto the last and smallest."
So it would also have been impossible for them to be let into Valinor as they did with Elves. Some of the hearts of Men would have been turned to evil, and sooner or later the Blessed Realm would have been betrayed to Morgoth. Not a desirable thing to happen.
But couldn't they at least have been protected or sent somewhere safe?
Now it wouldn't be fair to say that the Valar were completely negligent of the Atani. At the bidding of Manwë, Ulmo sent some Maia to visit them. But they never came visible among Men; they only spoke riddled messages with the voice of water, through streams and rivers. These messages weren't understood, and the only outcome of them was that the Atani had a love for water ever after. The Valar never lifted a finger to undertake anything bigger.
Think of what Men could be like. All races of Men, save those that used their gift of choice to evil purposes, could have been as noble and wise as the Númenóreans before their downfall. Rohirrim, Breelanders - even the Wild Men of Ghân-Buri-Ghân. If the Valar had desired it.
So the question arises: Were the Lords of the West biased in favour of Elves, or was all this actually the design of Eru himself?
The Valar just didn't understand Men enough as they understood Elves; Manwë himself didn't understand the part in the Music where Men came.
Anyway, at least after the War of Wrath they gave them a longer life and the isle of Numenor so they made up for their 'negligence' i guess.