Richardson and Rohach, a redshirt freshman, were originally listed as co-starters on a depth chart released Monday.
"The No. 1 reason is just the knowledge that he has overall, of pre-snap and post-snap," Rhoads said. "Sam's knowledge is just far ahead, quite simply, of where Grant's knowledge is. That's just time and experience."
Richardson, a redshirt sophomore, has started all six games for the Cyclones (1-5, 0-3) despite injuring an ankle in the opener. He's thrown for 1,255 yards and 10 touchdowns.
He was just 15-of-38 passing in an Oct. 12 loss at Texas Tech. On Saturday, Richardson was 7 of 14 for 68 yards and an interception as the Cyclones were trounced 71-7 by Baylor.
It got so bad in Waco that Rhoads went to Rohach for the first time. He was 8-of-13 passing for 65 yards, with a touchdown pass and an interception against Baylor's reserves.
"We plan on [Richardson] playing at a higher level with his physical health. He's got to make smarter decisions and quicker judgments," Rhoads said.
The Associated Press contributed to this report.