Schlitter has been on the DL since August 10 with shoulder inflammation. The day Schlitter hit the DL was the same day the Cubs added Jacob Turner to the roster, so there was always going to be a crunch when Schlitter was ready to return.
Recently, however, Edwin Jackson went on the DL with a lat strain, and the Cubs added lefty reliever Zac Rosscup to the bullpen. The Cubs could, then, have sent Rosscup back to Iowa to make room for Schlitter. It likely doesn’t matter a whole lot, being that rosters expand to the full 40-man roster in just over a week, so Schlitter (and Blake Parker, Chris Rusin, Dan Straily, Felix Doubront, and maybe Arodys Vizcaino) could be back very soon, and Rosscup would have come up anyway.
The Cubs probably didn’t want to go with just one lefty in the bullpen right now, so Rosscup stays.