Cuba Gooding Jr. was one of the original choices to play the role of Jim Rhodes/War Machine in Iron Man, but he was ultimately replaced by Terrence Howard. There are a few reasons why this change may have occurred.
For one, Cuba Gooding Jr. is known for his comedic roles, while the character of Jim Rhodes is a more serious and stoic figure. It's possible that the filmmakers felt Gooding Jr.'s comedic tendencies would not be suited for the tone of the film.
Additionally, there were reports that Gooding Jr. was difficult to work with on set and had difficulty memorizing his lines. This likely created friction between him and the cast and crew, leading to his eventual replacement.
Ultimately, it seems that a combination of factors led to Cuba Gooding Jr.'s replacement in Iron Man. While he may have been an initial choice for the role, it became clear that he wasn't the right fit for what the filmmakers were looking for in their adaptation of this popular comic book character.