Magill doesn't have minor league options. They are milking his rehab stint, because once they activate him, he has to stay. Romero first then Magill is possible - Magill then Romero isn't. And if Romero succeeds, there will be some other candidate to demote when Magill has to be activated.
Always do the possible, first.
I don't even think it's that complicated, in this situation.
They really need someone capable of pitching length tonight, Wednesday night, to back up Stewart.
Romero hasn't pitched since Saturday, and has had a full healthy spring/season so far. He could almost certainly throw 30-40 pitches tonight, if necessary.
Magill had an abbreviated spring, and just threw a season-high 26 pitches (2 innings) for Rochester on Monday night. They probably don't want to push him beyond that in his MLB season debut.
They might switch out Magill for Romero or Harper as soon as Friday.