Nihilus could devour a planet's lives, Palpatine's Force Storm could destroy a fleet in seconds, and Sauron could not even defeat a bunch of human and elves with cold weapon with an army on his hand.
Stormtroopers couldn't defeat a bunch of Teddy Bears with Sharp Spears and Bows. Boba Fett the best bounty hunter in the Galaxy got knocked in the Sarlacc pit by a Half Blind Man with a Stick. The Deathstar got blown up by a Farmboy with no Combat training. Lando Calrissian was able to fly a FREIGHTER into the tight and narrow Death Star II Tunnel and make it out. The Trade Federation Control Ship got blown up by a kid with no real flight training and no idea what he was doing. Darth Vader got his limbs cut off because he jumped at Obi Wan when he could have simply came from the below. Why?! Cuz George Lucas said so.
You want to know why Sauron didn't win? Cuz F'ing Tolkien DECREED IT!