If selling old games in HD has anything, its told the venders, publishers and developers that people will buy it.
Microsoft could sell an HD Halo pack and it would sell by the bucket load so why offer BC and lose your self some 'free' revenue?
Why cause the outrage from existing 360/ps3 customers for a little extra revenue? It is more important to keep as many customers as possible from previous generation by providing painless migration from existing console to the next.