Basically, during the "Lost Cause" era, Texas needed a Confederate hero, and it didn't have too many good options. Shiloh was retconned into a Confederate Victory, and Johnston was rewritten as a martyred hero who died on the field of battle.
Well, he was a martyred hero, if you regard a traitor as a hero, and death by stupidity as martyrdom.
Kind of glad they picked Sidney over Sam Hood though. Sam Hood as the Lost Cause Hero of Texas is stomach turning.