Those Crazy Ainur...

[Legolas]

I'm sure Tulkas, Eonwe, Manwe, and Ulmo would've been able to take Sauron, but why would they?

Morgoth was a fight that belonged to the Valar. Sauron was a different matter. Sure, the Valar could've sent their forces to dispose of Sauron quickly, but why?

Middle-earth had to prove itself. The Istari were sent to guide Middle-earth; to impart wisdom - not to face-off with Sauron and do all of the fighting for them. Middle-earth would've gained nothing, and another evil would've arisen for them to contend with. Middle-earth cries to Aman. The Valar save the day again. Another evil comes. Middle-earth cries again...get the point? If the Valar had to save the peoples of Middle-earth, they'd never learn the importance of sticking together and keeping hope. There'd be no point in Middle-earth existing if the people there never learned how to defeat evil.