But remember the Gazelle was used in the military as a trainer to prepare for more complex and faster operational types (in most cases) which almost always had a surplus of power - it was great trainer in that respect and a brilliant aircraft to fly (I'm still rated on it).
Since the military had no piston helicopters from the 70s onwards, why train on something like a Bell 47 just so you can learn overpitching and severe power limited techniques (none of which are exactly difficult to master)?