The reports prior to the trade deadline was that teams wanted too much for top closers. If the closers available only had a game or two advantage compared to Kimbrel the rest of the season, and the asking price as rumored was a top prospect or two, well, I can understand not pulling the trigger.
I'm not crazy about Gallo, but the price was a prospect they no longer believed in at least. There is a small but real chance of him having a real short-term streak that helps the Dodgers significantly.
I think the Dodgers have decided to try the youngsters if they can't get a superstar, reboot the salary cap, and go for superstars later on.