From what I heard, Jhonen got tired of it. I also heard Nickelodeon was being jackasses about it saying they wanted the show to continuously focus on the Invasion instead of silly alternative plots like the Dib episodes and stuff. I used to talk to Wally Wingert about the shows cancellation. He said it's highly doubtful it will return and if it does, the voices won't be the same most likely.
In every episode from a certain episode on, there's a hidden image of a bloody looking GIR. To this day only two have been found. It's transparent and only a couple frames long, so it's barely noticed. They weren't allowed to use it when it was airing, so the episodes that air on TV don't show it, but if you buy the DVDs you can see them (I think)