Why can't I play Forza Motorsport 3 on Xbox Series X?

I thought they pushed it back since they still haven't announced a release date

Which for this year should be done like now at the last xbox showcase like in april

Because they've already made newer versions of the game and are usually already working on more so the developers wouldn't want to put money into an older game.

Because the cost of renewing licensees, and this is the answer for virtually any game based on outside IP. Sports, Movies, Music etc etc, is far more than optimistic projections of additional sales

If they think a game like that has a big enough market to justify that, they tyipcally remaster it.

The other Forza Motorsport games are not available either unless you pick up a disc copy for the same reasons. A new Forza Motorsport is coming later this year

Why not renew the license and put the game for sale in the store doesn't make sense tbh

The game was never backward compatible. Probably due to licence agreements having already expired. The only way to continue playing the game, is to purchase another Xbox 360. The list of backward compatible games can be found here https://www.xbox.com/en-GB/games/backward-compatibility