I think the issue is that the 5C doesn't have the same "sleek and sexy" feel to it as you see on the other devices. But as pointed out already, granted that it is colorful and it is plastic in the rear, you tend not to want a case on it as it makes it even bulkier. The one turn-off for it I would say would be that the notion of "plastic breaks easily" plants a stigma right off the bat. Considering not that many users have actually had their hands on an iPhone 3G or 3GS, and most have started on the 4/4S if not later, the plastic feels off compared to a more firm, solid, sleek appearance.
Truthfully, the chance of a glass rear chassis on say a 4S breaking from a drop is the same as that of the 5C if you drop it on the right angle. Samsung has been using a thick plastic polymer since the beginning of time and I've yet to see someone actually break their phone unless they purposefully or accidentally sat on it with their rear pocket, which will break or crack and phone for that matter. The intro of the 5C is a good change to the iPhone line-up, but the impact may take a while to kick in. The ultimate goal of the 5C was cost, which it fulfilled (for carrier discounted prices) to a certain extent, but because the full MSRP differs about $100 at all stages of the game, and the 5C is technically using older hardware, the 5S somewhat outshines the 5C with the fingerprint reader, new 64 bit processor and M7 motion processor as well. If Apple were to release the 5C with the 5, I would say the 5C would catch on fairly well and by transitioning out the 5, the 5C wouldn't be as alienated as it is at the moment. But that's just my opinion
