Isto é respondido pelos próprios irmãos Russo:

What was the decision making process to bring Red Skull back?

Joe Russo: Somebody had to be the gatekeeper to the Soul Stone and somebody had to distribute the rules to Thanos and Gamora.

Anthony Russo: In a way that was credible and believable.

Joe Russo: Who felt credible to the audience. And a character who has been trapped – the Red Skull has been turned into this wraith who now guards the Soul Stone and distributes the rules to whoever shows up there trying to obtain it. It felt like there was more credibility than just a random character.

Anthony Russo: Somebody who has a history with the stones may seem like they have a little more authority in terms of their experience – especially a tragic experience. - uproxx.com

Então parece que eles escolheram alguém que tinha uma história com pedras infinitas. E lembre-se de que Red Skull é um personagem recorrente famoso em quadrinhos da Marvel e nós realmente não vemos o suficiente dele em comparação com seus arcos de quadrinhos.