There may be other reasons
There is a good reason. Probably, they haven't decided what pitcher should be the one to be sent down when they bring up Strasburg, and a premature announcement would possibly commit them to a choice, and the affected player would be served notice. Not especially good for morale, to be told your being replaced, but by the way, go out there and give it your best, even if it isn't going to save your ass from being sent down.
The Stammen / Atilano debate is far from settled, and Olsen needs to show he's got something left in that shoulder. All three are candidates.