In addition to what Andyb said, it is easier for a minor league pitcher to develop his pitches and command when pitching a starter's innings than when pitching a reliever's innings. Also, a SP just has so much more value than a comparable quality reliever that a team would be foolish to give up to soon on a guy with a chance to become an MLB starter.
Now, if the view is that a guy has a chance to be an iffy #4/5 starter but can be a plus, late-game reliever, then of course you convert him and an extra year of minor-league relieving may well launch him faster as an MLB reliever. This is where Andyb's post comes in: McGarry doesn't look like a guy who would be much better in relief (has enough pitches to start, doesn't tire quickly). If the reasonable ceilings are #3/4 starter and decent middle reliever, then the choice is clear.