Also, the fall of reach is supposed to the the ultimate low point in the 25 year war. The straw that broke the camels back, the moment when humanity knew it was utterly doomed, even though they has been thinking it for 25 years. All of humanities hope for at least a draw, let alone a win in the war died with reach. Not only were 60% of the UNSCs ships destroyed, but seemingly all but one of its most powerful ground assets (spartans) were destroyed too. Their deaths make reaches fall that much worse.
On the covenant side, they had been searching for the halo rings and they had been getting closer and closer to finding one. Then the artifact on sigma octanus led them to reach (which would've led them to halo if Halsey hadn't discovered it first). So naturally they are going to commit all their resources to getting the coordinates since this is what they had been waiting for since the covenant began. This explains why the fleet attacking reach was so damned powerful: their greatest hour was at hand and they knew it.
It makes the discovery of halo 100000000000000x more significant. The covenant had been killing off humanity for 25 years in the search for it, so deciphering what it did/why the covenant wanted it was the last hope humanity had.
Pity the games never properly covey just how desperate humanities situation is.
Thank god for nylund.
[Edited on 02.23.2011 8:02 PM PST]