Why should he join the Akatsuki organization?

No one knows exactly why they should have joined, but the possibilities are:
- Itachi uchiha teamed up to let Sasuke Uchiha know his location to face him in battle.
- The closeness with Akatsuki's leader Madara Uchiha, since Madara planned something mysterious with Sasuke or something, which led Sasuke to join the clan, one of the things that happens to me is that Madara somehow forced itachi to join akatsuki to obtain the bijjus.

- Although the third hokage most likely will tell him after the slaughter of his clan is that he join to collect information, although that should cost him his life and be considered a comfortable enemy