The decision to play giach over marrone is defintely not solely footballing reason.
Giach slogged his heart out for juve last season, along with other players to win the Serie A. how will he feel if he loses his substitute role to Marrone? There is a hierarchy in the ranks and Conte wants to respect that in order for the players to feel appreciated and commit to giving their all in games. This is how a great club works. In the long run, it will be beneficial to Juventus's reputation among footballers too.
Marrone will definitely see more playing time next season, or maybe even in the second half of the season when Juventus is in the knock-stage of CL, which I WISH THEY WILL BE!