Northerners and southerners contested the meaning and legacy of the Civil War from the moment Lee and Grant concluded their meeting at Appomattox on Palm Sunday, 1865. We tend to remember that moment as a two great leaders making a gentlemen's agreement to begin national reconciliation. In reality, both had agendas from the beginning. Lee promoted a narrative where he and his men had fought bravely for a noble and just cause and had surrendered only to avoid further bloodshed in the face of overwhelming odds.

Lee’s view, in short, was that the defeat was one of might over right, and that the South had maintained its honor. Grant felt that the Union “triumph flowed from the superior virtue of its cause.” Lee longed for a lost past, in which the southern aristocracy held a position of power and prestige in the United States. Grant looked forward to a future where the victorious North would continue to progress, hand in hand with a South which had learned its lesson. Former Confederate supporters and Copperhead Democrats supported Lee’s view, while Republicans, southern unionists, northern blacks and southern freedmen agreed with Grant.

The northern flavor of Civil War memory may be called "Union victory culture." Its southern cousin is known widely as the Lost Cause. Both of these visions competed in the first decades after the war, when the war generation on both sides commemorated their efforts and sacrifices. Union victory culture, however, pretty much died out with the Union veteran generation. By the turn of the 20th century, northerners largely lost interest in commemorating their victory, as they were more concerned with anxieties about economic growth, labor unrest, and increased globalization.

Civil War memory remained very important in the South because southerners had to come to terms with their defeat and the widespread ruin brought on by the war. They crafted narratives of victimhood and rationalized the rebellion by erasing slavery as its root cause.

Southern participation in the Spanish-American War gave the South new war heroes who fought under the old flag, including former Confederate generals Fitzhugh Lee and Joseph Wheeler. In 1905, the U.S. government returned captured Civil War battle flags to southern states, which southerners accepted as an affirmation of their martial honor. As the Civil War generation died out, custody of Confederate memory passed into the hands of organizations such as the United Daughters of the Confederacy and the Sons of Confederate Veterans. Essentially, the Lost Cause ceased to be the southern vision of the Civil War and became the national vision largely because northerners stopped caring enough to contest it.

This was the state of Civil War memory in the 1920s. I haven't seen the movie you mention, but my educated guess is that it wasn't so much that American audiences wouldn't accept Confederates as villains. It's more that the entire country would accept Confederate protagonists, but the South would absolutely reject Confederates as villains.

(NOTE: My response to this question is largely adapted from a comps answer I wrote on Civil War memory)

Sources:

Elizabeth R. Varon, Appomattox: Victory, Defeat, and Freedom at the End of the Civil War

David W. Blight, Race and Reunion: The Civil War in American Memory

Gaines M. Foster, Ghosts of the Confederacy: Defeat, the Lost Cause, and the Emergence of the New South