I think they just overestimated the demand for them. I think even though they can advertise the new 5C a lot of customers tend to know or understand that the 5S is the actual new phone. So while the 5C appears to sell better than the 4S did after the 5 release people still really want the 5S.
Especially since the cost difference between a 5C and 5S isn't really that much in the grand scheme of things.
Especially since the cost difference between a 5C and 5S isn't really that much in the grand scheme of things.