Most likely due to difficulties with licensing. I know they've said there were games that were a challenge to license just to get on back compat, and that's simply making a game that can already be purchased available on a new system. Getting those games where the rights holders may be convoluted to agree to appear on a subscription service, and the payment structure that entails, is probably even harder.
That makes sense when it comes to third-party games, but what about stuff like Blinx or other Microsoft properties?