Comparing Nashville to other cities is very informative. We just returned from Park City, UT where we took the bus daily. We walked everywhere. Our rental car was parked on day 1 and not revisited until our time to go.
One reason the bus was so highly utilized is its frequency. If you miss a bus, it frankly is not a big deal as there will be another in roughly 10 minutes.
A bonus is that it is a free service. This is not a requirement for use but it certainly was generous and we appreciated it as guests.
Another reason we felt it was easiest to use the bus with our family of five, including small children, was that we always stepped off onto a well connected sidewalk network so we could finish our trip safely.
By contrast, many bus stops in Nashville leave you stranded on a small concrete pad. There is no connection to the area - no place to walk safely. This presents a psychological barrier to use. Using the bus becomes a challenge rather than the easiest thing to do.
As many Nashvillians note bitterly, traffic is building. Until a well connected sidewalk network is in place, Nashville cannot expect increased ridership on its buses.