[Mithalwen]

My feeling is, and it is confirmed to an extent by what happened with Saruman, that the Valar had realised that the more inherently powerful a person was the more vulnerable they are to be corrupted by the possibilites of that power. They might start off with the best of intentions -then maybe do morally questionable things to provide a good outcome and end up no better than the Dark Lord.

Think of how frightened Gandalf was to have even indirect contact with the Ring and how tempted (but self aware) Galadriel was. Saruman started out like Gandalf but more powerful, he ended up setting himself up as a tyrannical power, an imitation Sauron. I think that they realised that they needed to incite or inspire the people of Middle Earth to overcome Sauron rather than to send a being of like status who would perhaps end up a replacement. That was Gandalf's role and he succeeded not only with the Hobbits but with Faramir and rousing Theoden.