The shortstop position has been a black hole for the Braves this year. The team is getting markedly inferior offensive production from that position as compared to any other on the diamond. Additionally, Tyler Pastornicky was a defensive liability, one whose poor play in the field is at least a partial explanation as to how the team is below average defensively despite having an above average defensive outfield. In short, the shortstop spot could be the difference between this team making the playoffs or having another "close, but no cigar" feeling in September.
So why make a move now? While I'm sure that the eight-game losing streak had something to do with the timing, a major reason would have to be that the Braves need to know whether they have an in-house solution to their problem. Pastornicky got a two-month audition and showed that he is not ready for prime time. The trading deadline is two months away. Doesn't it make sense for the Braves to give Simmons the same audition before they decide whether to look outside the organization for salvation?
Let's assume that Simmons gets off to a fast start at the plate. Don't the Braves want to see him go through the league a time or two so they can figure out whether he can adjust to the adjustments that pitchers will make in facing him? Conversely, let's say that the pressure of being cast as the team's savior is too much for the guy. Don't the Braves want to give him as many games as possible to settle into his role?