A possible lead....?

[Kwalitee]

Well wasn't there a Forerunner object on Reach?

(not disagreeing, just pointing out)

[ODST Epsilon]

Well wasn't there a Forerunner object on Reach?

(not disagreeing, just pointing out)

They knew it's location and avoided that area. We have no evidence (as of yet) that the Covenant know the Ark's location and that they've even commenced orbital bombardment.

[Orpheus]

yeah but in Ghost's of Onyx
[spoiler]the Covenant were doing stuff all over the world, including stuff in Antarctica and The Gulf of Mexico[/spoiler]
so maybe there's other stuff they need to find(including the ark) and they're spread out so far around the world that they don't risk destroying some of the aforementioned stuff

[failedparachute]

Well, from the trailers, as well as Regret's landing in New Mombassa, they've got a pretty good idea where it is. Where they found that information, it's been stated that it's likely that the Covenant procured the coordinates of Earth from another artifact. As for commencing orbital bombardment, they still need to get through the Super MAC guns as well as the UNSC Fleet, though that being Regret did get through, but I don't think it's near a big enough hole to let the armada through, especially with the fracturing of the Covenant fleet into 3 factions, the Brute controlled ships, the Elite controlled ships, and the Flood controlled ships. Chaotic, huh?