When the Ori had 7 ships above earth they just stood there until the other story-lines wrapped up, as well as the entire time it took to take the Ark from one galaxy to another. Why would they do that?
Landry offers a few different guesses in his chat with the Prior. You can take your pick which you think is most likely, but to my mind the fact that the Prior mentions Earth's defences suggest that it's the topmost.
- The Priors of the Ori are overly cautious. Earth has shown itself to have formidable planetary defences in the recent past. The Ori would be especially wary of those since they're based on Lantean design.
- The Priors of the Ori may simply be delaying until they have overwhelming forces in place.
- The Priors may be experiencing an existential crisis. Their gods have just died and this may be causing them some level of anxiety and hesitation.
- Earth is the juiciest plum in the whole Galaxy. Their goal may be to simply threaten destruction from the sky in the hopes that its people will see sense.
LANDRY: Sure, you've tried to warn us a couple of times, but come on, you can do better than that, can't you? We've been a tremendous pain in your proverbial backside. You don't really expect me to believe that you—you've just been saving the best for last, do you? What's wrong? You guys are never speechless! I'll tell you what I think. I say this move reeks of desperation. You're not all that sure it's a slam-dunk.
PRIOR: Your defenses are no match for the might of the Ori, which shall rain down apocalypse upon—
LANDRY: Right, right. Now here's a news-flash, the Ori are dead. We killed them. Sent a weapon devised by an ex-Ancient through the Supergate and wiped them all out [beat] These beings you call gods, the ones who lied to you, manipulated you, for their own selfish, power-hungry reasons, are gone, and everything you think you're doing in their name is entirely without meaning.