You use an experienced coach instead of the nearest guy with a scrum master certificate because you better chance of success when you're guided by someone who cares, connects, knows the work system, and has been through a few transitions before. Empathy and experience are the stuff that good coaches are made of, yet either of them can become a trap.
Likewise, a consultant can succumb to prior experience and waste his engagements trying to recreate a past glory. A good coach needs to respect the context and personalities and experience base of each team and help them to do their best; doing otherwise leads to frustration.
However great the past may have been and how much you care about the team, your clients need a present-tense, temporary coach.