Why is Arkham Origins not purchasable online but Arkham City + Asylum is?

Hi, the game is not currently backward compatible. Only Arkham Origins is compatible, but there is a couple of Xbox One versions of the game in a bundle titled Return to Arkham which gives you Arkham Asylum & Arkham City or you can get the Arkham Collection
which gives you Arkham Asylum, Arkham City & Arkham Knight, all remastered and Xbox One X enhanced.

There is mention, in Arkham City, of events that occured during Arkham Asylum that sort of tie the two games together. It may not make sense to you if you haven't played the first game. Knowing what I know about Arkham Asylum and what I've played so far
in Arkham City, I suggest playing the Asylum game first since Arkham City looks to be a continuation of the story line set up by Arkham Asylum. Besides, Arkham Asylum was a fantastic game and shouldn't be missed by anyone imo.

Actually, Gotham is Arkham City, the reason why its called this is because Arkham Asylum is shut down and they just block off a portion of the city.

"Quincy Sharp, former warden of Arkham Asylum, has taken credit for stopping the Joker's Asylum takeover, and used the notoriety to become mayor of Gotham City. Deeming Arkham and Blackgate Penitentiary no longer suitable to contain the city's criminals
and insane, Sharp has both facilities closed and buys out a large section of Gotham's slums, converting them into an immense open-air prison named Arkham City"

Origins was never made available to purchase digitally. I'm not sure exactly why, but it was most likely due to it having 2 discs.

That was just a guess. I've never been able to find an exact reason why it was never released digitally. It's one of the games I wanted forever too so I kept an eye out.