It feels as though the games which are on Stadia right now are games which, with few exceptions, are games which Google paid a significant amount of money to during development to support Stadia as well as other platforms.

I'm guessing that if you look at an existing title along the lines of any of the ones you mentioned, the team has to balance between ongoing development of the game and porting it to Stadia, and they may just not have the resources or inclination to do the porting even when huge cheques are involved.