"Why did the beater bar go out of fashion?"
The 'agitator' with the metal beater bars, apparently flattened the pile on some carpets.
The Turbopower 'activator' was supposed to reactivate flattened carpet pile.
Remember also that the agitator body was metal (i.e. expensive). The activator, in comparison, was plastic moulded, thus cheaper to manufacture.
The very last models of Hoover Junior U1104 had the plastic 'activator' (circa 1990), rather than the traditional metal 'agitator'.