What kiefer said.
They would have to redo the entire netcode which they could if they really wanted to but I'm guessing their too busy with their future titles.
I know personaly I would buy Halo:CE with live capability day one and I know a bunch of other people that would too. Honestly I think they could sell over 2 million easy if the re-release Halo:CE with live. Probably over 3 million.
Fingers crossed that one day in the future they surprise their fans and give us back one of the greatest fps ever with live support.
The definitive evidence that video games turn people into mass murderers