As someone who still owns a car anyway, I essentially never ride the bus because the marginal cost of a bus trip is almost always higher than the marginal cost of the equivalent car trip (basically every time unless I have to pay for parking, which is exceedingly rare). The total cost of the car trip is higher, to be sure, but most of that is fixed cost that I'd have to pay anyway, so foregoing it doesn't actually save me money. I could bring the marginal cost of the bus to $0 by buying a monthly transit pass, but unless it could eliminate all my car trips and thus allow me to get rid of my car, it's still a net increase in cost and doesn't make sense.

Even if I want to take transit, because it's a straight shot to my destination or lets me avoid a bunch of rush-hour traffic or something like that, it's not worth paying $5 round trip fare when gas to go that distance is $1.

On the other hand, if the transit fare were eliminated, I'd use it for many trips even as a car owner. There's a huge potential for congestion reduction just by eliminating that weird dichotomy where economics basically forces people to either go all-in on a transit pass or all-in on driving.

(In reality, I bike most of the time, in part because its marginal cost is even lower than driving. But ignore that for the purpose of the above argument and pretend driving and transit were my only choices.)

Edit:

Now that I think about it, it would change my habits even more because I often avoid going to places downtown at all due to the lack of free parking. If transit were free, it would probably be really good for business over there.

Also, this cost disparity is exacerbated when you're talking about a family (or other group) instead of an individual. Spending $5 in transit fare instead of $1 in gas is bad enough, but when it's $20 in transit fare instead of $1 for my family of four, even the paid parking starts looking like a better alternative, which is terrible.