That's unfortunate. He had been recently wounded by an Allied fighter plane attacking his command car, but had been expected to make a complete recovery. Because the Western Allies respected him greatly due to his conduct of the North Africa campaigns, the conspirators wanted him to be the one who would approach the Western Allies with the proposed deal.
I suspect, however, that the reduction of Rommel's role in the movie had something to do with an insistence by Tom Cruise, or his agents. Compared to Rommel, von Stauffenberg was a nobody. A larger role for Rommel in the movie would have stolen some of his thunder.
Sounds like a good explanation. I agree that Stauffenberg should not be diminished since many people have no knowledge of the plot(I suspect that many don’t know there was a World War II, but that is for another discussion)