The Red Bull organization/technical team saw something in Marsch. They fired Petke, a young coach who was doing well, after 2 seasons. After Marsch 2nd season, they had him doing his UEFA Pro license, and as soon as he was completed, they hired him as a RB Leipzig assistant coach.
Mike Petke is pretty good - his teams usually play high pressing & possession-based soccer, likes technical & skilful players, and isn't shy about playing young players, BUT humility isn't in his vocabulary. That's probably what turned off the Red Bull organization.
It will be interesting to see who will be the leading USMNT head coach candidate, for the 2026 World Cup. Marsch or David Wagner (who I hear is getting interest from England & Germany/Bundesliga).