The downside to providing an extensive service is, of course, cost. It's no secret that Metro (government owned) loses a lot of money in order to provide bus services. But to be fair, that loss is largely because 80% of the people riding on buses aren't paying full fare, they are students, pensioners, unemployed and so on eligible for a concession.
So it basically comes down to welfare provision. Commuter services for city workers make a profit since just about everyone on board would be paying full fare and the buses would normally be quite full. But other services are either running late at night with very few people on board or are primarily used by those who don't pay full fare. As such they lose money. But from a broader social perspective, I'd argue that there's a need to provide services of an evening, weekends and so on even if they aren't profitable. Hence public ownership rather than private makes sense given that making a profit is not the intent.