I'm afraid I think the C5 misunderstood the market for transportation devices completely. It wouldn't sell today, either, because people look ridiculous in one, and because it fails to offer people the comfort they expect from a car. One of the main drivers of automobilism, for instance, is the ability to travel together and have conversations and social interaction, which the C5 didn't offer. THE main driver of automobilism is the nimbus of it being a universal tool fit for use in all transport scenarios (which it obviously isn't, just as computers should be used for far fewer applications, but the nimbus exists).
The C5 actually has quite a few similarities to (still very niche) recumbent velomobiles. I don't think the lack of mass attractiveness of this concept will change in a hurry. Maybe when half the world has been destroyed by catastrophic flooding.