Because you have other guys in the bullpen desperate for pitching time in low leverage situations. The Cubs need to capitalize on those. Besides, it's usually not a bad idea to make sure the opposing team doesn't have a chance to adjust to a reliever if he's given you a good inning, especially if it's a pitcher without overwhelming stuff. It's not like he had to go back out there because the bullpen was taxed or because the lead was close. In short, there was no need to send him back out there. He's a reliever, he got you your three outs, you have other guys that need to pitch; pick someone else.