Endgame. The Soul Stone manipulated the Avengers into sending Nat and Clint to retrieve it.

The obvious team to go to a dangerous, almost unknown world would be Hulk, the super strong super scientist, and Rocket, the expert space pilot and weapons master. Nat is the acknowledged expert on manipulation and persuasion, much better to talk to the Ancient One. Clint would have been fine going with Thor.

The only reason to send Black Widow and Hawkeye was to force them into making the choice.