Aside from the fact that he was blocked by Vlad/Belt this season, Horowitz on July 1 was hitting .297/.419/.390 in AAA. For a 1B/DH, that wasn't good enough to call up on a team trying to contend. In July/Aug, he hit .394/.498/.644, so the power for whatever reason started to develop and he became a more interesting prospect. Now was probably the right time to call him up.
I could definitely see Schneider and Horowitz replacing Merrifield and Belt next season. Maybe Orelvis has a chance at 3B too, but that's a lot of rookies for a team that will likely be very desperate to make the playoffs, so they might prefer the stopgap route.