User blog comment:Thailog/Ep. 202 "Earthlings" discussion/@comment-128.54.156.144-20120506165637/@comment-173.10.234.230-20120506200854

The Guardians don't know everything, and they will know more if they had a lantern in the sector. It may be that there was no lantern assigned to the Rann Sector. Just like we see no Thanagarian Lantern, the people of Rann may be territorial enough to prevent interference from the Lanterns. Plus the Gardians weren't informed enough to stop Sinestro when he went rogue. They didn't even expect it.