Darth Plagueis was a Dark Lord of the Sith, so powerful and so wise he could use the Force to influence the midichlorians to create life… He had such a knowledge of the dark side, he could even keep the ones he cared about from dying. He became so powerful… the only thing he was afraid of was losing his power, which eventually, of course, he did.

Basically if they had attempted to fly in earlier, they would've been seen by the eye and then utterly demolished by the still strong forces within Mordor as Sauron was still very powerful and his armies were definitely a force to be reckoned with. That being said it is conceivable that a mission that bold could've been completed if the elves had also gotten involved, but they didn't care too much for the whole ordeal.