Then why did the government mandate passive systems
such as the "motorized" seatbelts of the early 90s
prior to driver side and then passenger side air bags?
Because some people felt that the government should think on behalf of citizens,
instead of letting them think for themselves. I tend to prefer individual
freedom of thought and action, myself.
Could it have been because voluntary use of
seatbelts wasn't sufficiently high?
Sufficiently high for whom? And if it's not "sufficiently high," what makes it