My guess is that because a gondola is so similar in size to a car, that the expectation is there for it to feel like a more private mode of transportation? I mean, yes, you've got Uber, Lyft, and taxis, but generally it's accepted that chit-chatting with the driver isn't necessary as they are providing a service, so those situations tend to lack awkwardness as there are pre-established roles. I would think sharing a gondola with strangers would feel similar to sharing a cab with a stranger - which is not something everyone would be comfortable with.