I wouldn't necessarily say that the fad has ended. I would say that since fitness trackers were introduced several years ago, early adopters were quick to realize the limitations of them. However with marketing/advertisements, they became more widely visible and available to the main market. A new pool of consumers opened up but came to the same realization of the poor quality and commoditization of step counters/fitness trackers.

I see it as some combined effect that is manifested in GoPro/FitBit stock (i.e. tanking). The reality is that the average person doesn't do any amount of extreme activity such that the step counters provides any additional value. But the lifestyle aspects shown in the ads convinces people that getting one will change their lives.