The Redbirds (10-1, 7-1), whose only other title came in 1999, started quickly with Robertson hiting Cameron Meredith for a 30-yard touchdown and Lechein Neblett for a 70-yarder before the Redbirds adding a safety in the first quarter.
Roberson finished 14 of 23 for a career-high 319 yards. Meredith had a career high of 182 yards on eight catches, his third-straight 100-yard game, and Neblett had four catches for 125.
Coprich's two touchdowns gave him a school record 21 this season.
Southern Illinois (6-6, 3-5) got three touchdown passes from Mark Iannotti.
Illinois State had 602 total yards.
