It has never been implied that Sasuke couldn’t Naruto 689 control Susano’o in the beggining. He used a Perfect Susano’o (to protect Kurama) all of a sudden, without any training.
You wonder why Itachi or Sasuke didn’t used the Perfect Susano’o before when it was necessary? For the same reason Sakura didn’t used the Byakugō seal.
Itachi needed to be resurrected. Obito? B!tch please! He can do sharingan jutsus using spiritual sharingans baby! And he can pass on his powers to the physical world like Hagoromo! Kakashi? Naah. Inutile as Sakura. Could do nothing until Obito gave him one last loving and tenderness!
This is chapter is Obito’s show. Kakashi is nothing more than a beneficiary of that show.Or like hiruzen said while fighting orochimaru in part 1, the sealer and the sealed are forever looked in eternal combat in the death god. Wouldn’t make sense if he just achieved it after edo tensei because using the version he used requires cooperation from kyuubi. And I doubt the first thing kyuubi will do after getting back is help the one that sealed him. Kyuubi is a hateful creature and that’s the darker half in naruto so Minato couldn’t have just achieved this now, or even while he was alive. Supports the theory about death god belly just being another dimension.