One problem simply is, it would be great for 360 owners if every possible title would be released at once but it would be dumb for publishers.
They benefit from selling digital copies with basically no effort.
Now just for example, we have five last gen only Ass Creeds and eight last gen only CoD's. Flooding the shop with all titles at once basically leads to people buying less overall. Some might pick Black Ops, others Modern Warfare 2, again others Black Ops 2... and then everyone is more or less fed up.
Releasing one game at a time gives every single game the full attention.
And yes, there's all the usual suspects like licensing issues for comics, music, movie games... and possibly publishers that don't want those digital releases. Especially if there's a remaster.