Only guessing But could it be to do with keeping the main pitch in better nick, or anything to do with staging costs? In days of yore when reserve matches were evening affairs, the main floodlights were needed for most (all) of the year. Now they're played in the afternoon and these are no longer necessary there may be some advantages to using the academy pitch.
The reserves do play on Oakwell,there was just a few games last winter when they played on the academy ground to save the main pitch for league games.