This is simply economy of scale.
You don't bring out your biggest guns onto a hardware base of only a few million units sold. Hence you have to bridge the waiting period somehow.
MS is doing it by throwing old games into the ring. At what costs would be interesting to know. It's certainly costing a lot of manpower. And I highly doubt this old stuff brings new customers into the fold, it certainly retains old customers until Halo Version 2345, ForzaVersion 7629, Gears Version 2109 are ready.
Sony is in the position that they have just enough manpower in studio output so they can throw some "not-quite-bigges-gun" material into the ring and not waste their limited budget and manpower on old stuff - as nice as preaching "backwards compatibility" to the choir may sound.