The waiver trade deadline runs through August. IF a trade is made, those spots given to those minor league non-40 man roster players may be needed. While other moves could be made, it would simplify things if you didn't have to contend with working around moves that didn't have to be made or decided yet. Additionally, if you promote them to the major league team and then acquire a player before the waiver trade deadline, you would then have to option that player back to the minors, using an option year. If you waited until September you could keep the player on the team without wasting an option.
It only uses an option if they are down 20 days or longer.
September is only 15 days away, so there is no longer a risk of burning an option.