There are a variety of reasons. Unlike many of the Confederate officers, Longstreet was from South Carolina, not Virginia, and the Lost Cause was quite Virginia-centric in how it viewed the War. Certainly, many of the Confederates who created the myth were Virginian, like Gen. Jubal Early. And they were also quite devoted to the memory of Robert E. Lee. It was Lee who was responsible for the defeat at Gettysburg, but Jubal Early and others tried to deflect the blame onto Longstreet, saying that if he had been able to attack early in the morning of July 2, the battle would have been won. That was at best very dubious, given Lee's determination to repeatedly attack a well-fortified Union position and his ordering of Pickett's disastrous charge. But factually untrue was their charge that Longstreet had actually disobeyed Lee, purposefully delayed out of spite for having his opinions on tactics for the battle dismissed. This was embroidered for more effect by Douglas Southall Freeman in his worshipful multi-volume biography of Lee.

Worst, Longstreet became a model reconstructed Confederate. He went through the process to get pardoned, called on other Confederates to accept Federal laws and Reconstruction, even joined the Republican party, and served as a Federal bureaucrat. While he did not become a Radical Republican, remained a conservative Southerner in many ways, he got on with his life. He was to many unreconstructed Confederates the pre-eminent "scalawag", a southerner who went to work for the North.

Lastly, Longstreet's papers were largely lost in a fire, so his biographers wanting to contest the Lost Cause have had to rely on comparatively few documents.

William Garrett Piston: Lee's Tarnished Lieutenant: James Longstreet and His Place in Southern History