From the sounds of it their architecture may not support this. If they had a SAN solution capable of replication to multiple data centers like HP LeftHand's product or a multiple master DRBD configuration they may be able to host github from multiple active datacenters and announce the block equally so that providers route traffic to them because their ASN is closest.
Who knows, maybe they do all of this?
Other than the added complexity, can you share any details about the cost? It seems like theoretically some managed hosting providers could offer this assuming they were in multiple datacenters, but I haven't seen any that do.