I have some thoughts but everything in this regard is highly speculative so we will never know a definite answer =)

Ok so you might know that after the war of wrath, bellerian got destroyed and sunk beneath the ocean. The Valar (or Eru? I am not sure) reshaped the world so that the Undying could never again be reached - except for a single route left open for the remaining elves. I am also not sure if this happened right after the war or only after the destruction of Numenor.

Anyway, the Valar were pretty much fed up with Middle-Earth at this point. A lot of elves had turned their back on them for the Silmarils, the Edain (Numenor) proved to be foolish and ignored the ban for setting foot into the Undying lands and payed the price of destruction.
This last time the Valar directly interferred in ME was only because Earendils plea for it and it was a costly war and destroyed a big part of the world.

And now again, even though having free will, a dark being gathered forces among the people of ME. How many time should they intervene to save them from themselves?

Also they probably knew that Sauron would never be a threat to them.

So they decided to send only the Wizards as help and only as a support - to help the free peoples, helping themselves.

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